ath9k: Turn ath_txq_lock/unlock() into static inlines.

These are one-line functions that just call spin_lock/unlock_bh(); turn
them into static inlines to avoid the function call overhead.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>