rt2x00: fix cancelling uninitialized work

{rx,tx}done_work's are only initialized for usb devices.

Signed-off-by: Stanislaw Gruszka <[e-mail]>
Cc: [e-mail]
Acked-by: Ivo van Doorn <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>

drivers/net/wireless/rt2x00/rt2x00dev.c

···
 	 * Stop all work.
 	 */
 	cancel_work_sync(&rt2x00dev->intf_work);
-	cancel_work_sync(&rt2x00dev->rxdone_work);
-	cancel_work_sync(&rt2x00dev->txdone_work);
+	if (rt2x00_is_usb(rt2x00dev)) {
+		cancel_work_sync(&rt2x00dev->rxdone_work);
+		cancel_work_sync(&rt2x00dev->txdone_work);
+	}
 	destroy_workqueue(rt2x00dev->workqueue);
 
 	/*