set NOT_BUSY host keepalive state at the beginning of loop

Firmware/Marlin_main.cpp

@@ -1388,6 +1388,7 @@ void host_keepalive() {
 // Before loop(), the setup() function is called by the main() routine.
 void loop()
 {
+	KEEPALIVE_STATE(NOT_BUSY);
 	if (usb_printing_counter > 0 && millis()-_usb_timer > 1000)
 	{
 		is_usb_printing = true;