Updates for stepper parity with 2.0.x

Matching changes from #8725

Marlin/stepper.cpp

   long Stepper::counter_m[MIXING_STEPPERS];
 #endif
 
-unsigned short Stepper::acc_step_rate; // needed for deceleration start point
 uint8_t Stepper::step_loops, Stepper::step_loops_nominal;
-unsigned short Stepper::OCR1A_nominal;
+
+uint16_t Stepper::OCR1A_nominal,
+         Stepper::acc_step_rate; // needed for deceleration start point
 
 volatile long Stepper::endstops_trigsteps[XYZ];
 
     NOMORE(acc_step_rate, current_block->nominal_rate);
 
     // step_rate to timer interval
-    const uint16_t timer = calc_timer(acc_step_rate);
+    const uint16_t interval = calc_timer_interval(acc_step_rate);
 
-    SPLIT(timer);  // split step into multiple ISRs if larger than  ENDSTOP_NOMINAL_OCR_VAL
+    SPLIT(interval);  // split step into multiple ISRs if larger than  ENDSTOP_NOMINAL_OCR_VAL
     _NEXT_ISR(ocr_val);
 
-    acceleration_time += timer;
+    acceleration_time += interval;
 
     #if ENABLED(LIN_ADVANCE)
 
           current_estep_rate[TOOL_E_INDEX] = ((uint32_t)acc_step_rate * current_block->abs_adv_steps_multiplier8) >> 17;
         #endif
       }
-      eISR_Rate = adv_rate(e_steps[TOOL_E_INDEX], timer, step_loops);
+      eISR_Rate = adv_rate(e_steps[TOOL_E_INDEX], interval, step_loops);
 
     #endif // LIN_ADVANCE
   }
       step_rate = current_block->final_rate;
 
     // step_rate to timer interval
-    const uint16_t timer = calc_timer(step_rate);
+    const uint16_t interval = calc_timer_interval(step_rate);
 
-    SPLIT(timer);  // split step into multiple ISRs if larger than  ENDSTOP_NOMINAL_OCR_VAL
+    SPLIT(interval);  // split step into multiple ISRs if larger than  ENDSTOP_NOMINAL_OCR_VAL
     _NEXT_ISR(ocr_val);
 
-    deceleration_time += timer;
+    deceleration_time += interval;
 
     #if ENABLED(LIN_ADVANCE)
 
           current_estep_rate[TOOL_E_INDEX] = ((uint32_t)step_rate * current_block->abs_adv_steps_multiplier8) >> 17;
         #endif
       }
-      eISR_Rate = adv_rate(e_steps[TOOL_E_INDEX], timer, step_loops);
+      eISR_Rate = adv_rate(e_steps[TOOL_E_INDEX], interval, step_loops);
 
     #endif // LIN_ADVANCE
   }

Marlin/stepper.h

     #endif // !LIN_ADVANCE
 
     static long acceleration_time, deceleration_time;
-    //unsigned long accelerate_until, decelerate_after, acceleration_rate, initial_rate, final_rate, nominal_rate;
-    static unsigned short acc_step_rate; // needed for deceleration start point
     static uint8_t step_loops, step_loops_nominal;
-    static unsigned short OCR1A_nominal;
+
+    static uint16_t OCR1A_nominal,
+                    acc_step_rate; // needed for deceleration start point
 
     static volatile long endstops_trigsteps[XYZ];
     static volatile long endstops_stepsTotal, endstops_stepsDone;
 
   private:
 
-    FORCE_INLINE static unsigned short calc_timer(unsigned short step_rate) {
+    FORCE_INLINE static unsigned short calc_timer_interval(unsigned short step_rate) {
       unsigned short timer;
 
       NOMORE(step_rate, MAX_STEP_FREQUENCY);
 
       deceleration_time = 0;
       // step_rate to timer interval
-      OCR1A_nominal = calc_timer(current_block->nominal_rate);
+      OCR1A_nominal = calc_timer_interval(current_block->nominal_rate);
       // make a note of the number of step loops required at nominal speed
       step_loops_nominal = step_loops;
       acc_step_rate = current_block->initial_rate;
-      acceleration_time = calc_timer(acc_step_rate);
+      acceleration_time = calc_timer_interval(acc_step_rate);
       _NEXT_ISR(acceleration_time);
 
       #if ENABLED(LIN_ADVANCE)