ath9k: Fix TX poll work locking

There is no need to call ath_txq_unlock_complete() in the
TX poll routine - frame completion is not done here,
so use ath_txq_unlock().

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