rt2x00: Don't kill guardian_urb when it wasn't created

This fixes a "BUG: unable to handle kernel paging request"
bug in rt73usb which was caused by killing the guardian_urb
while it had never been allocated for rt73usb.
Signed-off-by: NIvo van Doorn <IvDoorn@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00usb.c

@@ -362,6 +362,12 @@ void rt2x00usb_disable_radio(struct rt2x00_dev *rt2x00dev)
 		}
 	}
 
+	/*
+	 * Kill guardian urb (if required by driver).
+	 */
+	if (!test_bit(DRIVER_REQUIRE_BEACON_GUARD, &rt2x00dev->flags))
+		return;
+
 	for (i = 0; i < rt2x00dev->bcn->limit; i++) {
 		priv_bcn = rt2x00dev->bcn->entries[i].priv_data;
 		usb_kill_urb(priv_bcn->urb);