ath5k: remove _bh from inner locks

spin_unlock_bh(&txq->lock) already disables softirqs so we don't want
to do it here.  Fixes smatch warnings:

   drivers/net/wireless/ath/ath5k/base.c:1048 ath5k_drain_tx_buffs() error: double lock 'bottom_half:'
   drivers/net/wireless/ath/ath5k/base.c:1056 ath5k_drain_tx_buffs() error: double unlock 'bottom_half:'

Reported-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>