rt2x00: Don't perform watchdog checks on empty queue

The currently used watchdog functions cannot be applied
to empty queues.
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00usb.c

@@ -363,11 +363,13 @@ void rt2x00usb_watchdog(struct rt2x00_dev *rt2x00dev)
 	struct data_queue *queue;
 
 	tx_queue_for_each(rt2x00dev, queue) {
+		if (!rt2x00queue_empty(queue)) {
 			if (rt2x00queue_dma_timeout(queue))
 				rt2x00usb_watchdog_tx_dma(queue);
 			if (rt2x00queue_timeout(queue))
 				rt2x00usb_watchdog_tx_status(queue);
 		}
+	}
 }
 EXPORT_SYMBOL_GPL(rt2x00usb_watchdog);