Fix compiler warnings with USB Disk on Teensy 2.0

usb_disk/media_sdcard.c

 
 static void media_send_end(void)
 {
-	uint8_t r;
+	//uint8_t r;
 
-	r = sd_command(SD_CMD_STOP_TRANSMISSION, 0);
+	/* r = */ sd_command(SD_CMD_STOP_TRANSMISSION, 0);
 	// TODO: proper handling of stop transaction.....
 	// but what is the proper way?	Older cards stop instantly,
 	// but newer ones spew 1 or 2 bytes of garbage, then maybe

usb_disk/usb.c

 	uint8_t n, state;
 	const uint8_t *p;
 	static uint32_t tag, xfer_len, lba;
-	static uint8_t dir, cmd_len, status;
+	static uint8_t dir __attribute__ ((used));
+	static uint8_t cmd_len __attribute__ ((used));
+	static uint8_t status;
 	static uint8_t scsi_sense, scsi_asense;
 	static uint16_t sector_count;
 	static uint8_t sector_chunk;