Use lowercase names for serial instances

Marlin/stepper.cpp

@@ -655,7 +655,7 @@ ISR(TIMER1_COMPA_vect) {
     // Take multiple steps per interrupt (For high speed moves)
     for (int8_t i = 0; i < step_loops; i++) {
       #ifndef USBCON
-        MSerial.checkRx(); // Check for serial chars.
+        customizedSerial.checkRx(); // Check for serial chars.
       #endif
 
       #if ENABLED(ADVANCE)