ath: move spin_lock_bh to spin_lock in tasklet

as you are already in a tasklet, it is unnecessary to call
spin_lock_bh, because softirq already disable BH.

Signed-off-by: Zhiwei Jiang <qq282012236@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>