planner.cpp: Add FAN_MIN_PWM for slow fan stopping issue …

@CONSULitAS

planner.cpp: Add FAN_MIN_PWM for slow fan stopping issue (V2.1 with
macro and linear scaling)

@thinkyhead thanks for idea on linear scaling

Marlin/planner.cpp

         } else {
           fan_kick_end = 0;
         }
-    #endif//FAN_KICKSTART_TIME
+    #endif //FAN_KICKSTART_TIME
+	#ifdef FAN_MIN_PWM
+      #define CALC_FAN_SPEED (tail_fan_speed ? ( FAN_MIN_PWM + (tail_fan_speed * (255 - FAN_MIN_PWM)) / 255 ) : 0)
+    #else
+      #define CALC_FAN_SPEED tail_fan_speed
+    #endif // FAN_MIN_PWM
     #ifdef FAN_SOFT_PWM
-      fanSpeedSoftPwm = tail_fan_speed;
+      fanSpeedSoftPwm = CALC_FAN_SPEED;
     #else
-      analogWrite(FAN_PIN, tail_fan_speed);
-    #endif //!FAN_SOFT_PWM
+      analogWrite(FAN_PIN, CALC_FAN_SPEED);
+    #endif // FAN_SOFT_PWM
   #endif // HAS_FAN
 
   #ifdef AUTOTEMP