ath9k: Fix pending frame check

Checking for the queue depth outside of
the TX queue lock is incorrect and in this
case, is not required since it is done inside
ath9k_has_pending_frames().

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>