ath9k: fix tx queue sparse complaint

This fixes this rant from sparse:

  CHECK   drivers/net/wireless/ath/ath9k/xmit.c
drivers/net/wireless/ath/ath9k/xmit.c:107:13: warning: context imbalance in 'ath_txq_lock' - wrong count at exit
drivers/net/wireless/ath/ath9k/xmit.c:112:13: warning: context imbalance in 'ath_txq_unlock' - unexpected unlock
drivers/net/wireless/ath/ath9k/xmit.c:123:30: warning: context imbalance in 'ath_txq_unlock_complete' - unexpected unlock
  CC [M]  drivers/net/wireless/ath/ath9k/xmit.

Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>