rt2x00: fix usage of NULL queue

We may call rt2x00queue_pause_queue(queue) with queue == NULL. Bug
was introduced by commit 62fe7784
"rt2x00: Fix stuck queue in tx failure case" .

Cc: stable@kernel.org # 3.0+
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>