Add COUNT macro and apply

Marlin/stepper.cpp

@@ -1249,7 +1249,7 @@ void microstep_init() {
     pinMode(E0_MS1_PIN,OUTPUT);
     pinMode(E0_MS2_PIN,OUTPUT);
     const uint8_t microstep_modes[] = MICROSTEP_MODES;
-    for (uint16_t i = 0; i < sizeof(microstep_modes) / sizeof(microstep_modes[0]); i++)
+    for (uint16_t i = 0; i < COUNT(microstep_modes); i++)
       microstep_mode(i, microstep_modes[i]);
   #endif
 }