temperature: Fix SOFT_PWM off by one

A 128 step PWM has 127 intervals (0/127 ... 127/127 duty). Currently, a
PWM setting of 1/127 is active for 2/128, i.e. double the expected time,
or, in general n+1/128 instead of n/127.
Fixes issue#6003.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

Marlin/temperature.cpp

     /**
      * Standard PWM modulation
      */
-    if (pwm_count == 0) {
+    if (pwm_count >= 127) {
+      pwm_count = 0;
       soft_pwm_0 = soft_pwm[0];
       WRITE_HEATER_0(soft_pwm_0 > 0 ? HIGH : LOW);
       #if HOTENDS > 1
       #endif
     }
 
-    if (soft_pwm_0 < pwm_count) WRITE_HEATER_0(0);
+    if (soft_pwm_0 <= pwm_count) WRITE_HEATER_0(0);
     #if HOTENDS > 1
-      if (soft_pwm_1 < pwm_count) WRITE_HEATER_1(0);
+      if (soft_pwm_1 <= pwm_count) WRITE_HEATER_1(0);
       #if HOTENDS > 2
-        if (soft_pwm_2 < pwm_count) WRITE_HEATER_2(0);
+        if (soft_pwm_2 <= pwm_count) WRITE_HEATER_2(0);
         #if HOTENDS > 3
-          if (soft_pwm_3 < pwm_count) WRITE_HEATER_3(0);
+          if (soft_pwm_3 <= pwm_count) WRITE_HEATER_3(0);
         #endif
       #endif
     #endif
 
     #if HAS_HEATER_BED
-      if (soft_pwm_BED < pwm_count) WRITE_HEATER_BED(0);
+      if (soft_pwm_BED <= pwm_count) WRITE_HEATER_BED(0);
     #endif
 
     #if ENABLED(FAN_SOFT_PWM)
       #if HAS_FAN0
-        if (soft_pwm_fan[0] < pwm_count) WRITE_FAN(0);
+        if (soft_pwm_fan[0] <= pwm_count) WRITE_FAN(0);
       #endif
       #if HAS_FAN1
-        if (soft_pwm_fan[1] < pwm_count) WRITE_FAN1(0);
+        if (soft_pwm_fan[1] <= pwm_count) WRITE_FAN1(0);
       #endif
       #if HAS_FAN2
-        if (soft_pwm_fan[2] < pwm_count) WRITE_FAN2(0);
+        if (soft_pwm_fan[2] <= pwm_count) WRITE_FAN2(0);
       #endif
     #endif
 
     // 4:                /  8 = 122.0703 Hz
     // 5:                /  4 = 244.1406 Hz
     pwm_count += _BV(SOFT_PWM_SCALE);
-    pwm_count &= 0x7F;
 
   #else // SLOW_PWM_HEATERS
 
     #endif
 
     #if ENABLED(FAN_SOFT_PWM)
-      if (pwm_count == 0) {
+      if (pwm_count >= 127) {
+        pwm_count = 0;
         #if HAS_FAN0
           soft_pwm_fan[0] = fanSpeedSoftPwm[0] >> 1;
           WRITE_FAN(soft_pwm_fan[0] > 0 ? HIGH : LOW);
         #endif
       }
       #if HAS_FAN0
-        if (soft_pwm_fan[0] < pwm_count) WRITE_FAN(0);
+        if (soft_pwm_fan[0] <= pwm_count) WRITE_FAN(0);
       #endif
       #if HAS_FAN1
-        if (soft_pwm_fan[1] < pwm_count) WRITE_FAN1(0);
+        if (soft_pwm_fan[1] <= pwm_count) WRITE_FAN1(0);
       #endif
       #if HAS_FAN2
-        if (soft_pwm_fan[2] < pwm_count) WRITE_FAN2(0);
+        if (soft_pwm_fan[2] <= pwm_count) WRITE_FAN2(0);
       #endif
     #endif //FAN_SOFT_PWM
 
     // 4:                /  8 = 122.0703 Hz
     // 5:                /  4 = 244.1406 Hz
     pwm_count += _BV(SOFT_PWM_SCALE);
-    pwm_count &= 0x7F;
 
-    // increment slow_pwm_count only every 64 pwm_count (e.g., every 8s)
-    if ((pwm_count % 64) == 0) {
+    // increment slow_pwm_count only every 64th pwm_count,
+    // i.e. yielding a PWM frequency of 16/128 Hz (8s).
+    if (((pwm_count >> SOFT_PWM_SCALE) % 64) == 0) {
       slow_pwm_count++;
       slow_pwm_count &= 0x7f;