Suppress Teensy 4 USB stack GCC warnings

There were two warnings from the Teensy 4 core updates:

/usr/local/share/arduino/hardware/teensy/avr/cores/teensy4/usb.c: In function 'endpoint0_complete':
/usr/local/share/arduino/hardware/teensy/avr/cores/teensy4/usb.c:766:10: warning: variable 'setup' set but not used [-Wunused-but-set-variable]
  setup_t setup;
          ^
/usr/local/share/arduino/hardware/teensy/avr/cores/teensy4/usb.c: At top level:
/usr/local/share/arduino/hardware/teensy/avr/cores/teensy4/usb.c:98:16: warning: 'endpoint0_buffer' defined but not used [-Wunused-variable]
 static uint8_t endpoint0_buffer[8];

Both of these are from not reading the endpoint 0 data in the `endpoint0_complete()` function. Reading and casting to `void` suppresses the warnings and should be optimized out by the compiler.

Author: dmadison

teensy/avr/cores/teensy4/usb.c

@@ -822,6 +822,11 @@ static void endpoint0_complete(void)
 		usb_audio_set_feature(&endpoint0_setupdata, endpoint0_buffer);
 	}
 #endif
+#ifdef XINPUT_INTERFACE
+	// dummy reads to suppress '-Wunused-but-set-variable' and '-Wunused-variable' warnings from GCC
+	(void) setup.wIndex;
+	(void) endpoint0_buffer;
+#endif
 }
 
 static void usb_endpoint_config(endpoint_t *qh, uint32_t config, void (*callback)(transfer_t *))