ar9170: fix read & write outside array bounds

queue == __AR9170_NUM_TXQ would cause a bug on the next line.

found by Smatch ( http://repo.or.cz/w/smatch.git ).

Cc: stable@kernel.org
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>