Fix OCR / PWM calculation (#18094)

Fixes #17968

Marlin/src/HAL/AVR/fast_pwm.cpp

     else
       top = *timer.ICRn; // top = ICRn
 
-    _SET_OCRnQ(timer.OCRnQ, timer.q, v * float(top / v_size)); // Scale 8/16-bit v to top value
+    _SET_OCRnQ(timer.OCRnQ, timer.q, v * float(top) / float(v_size)); // Scale 8/16-bit v to top value
   }
 }