ath9k: Do not use local_bh_disable in ampdu_action

This was added during the early conversion of ampdu_action
to a sleeping callback. There is no need to do this - instead,
use the normal mutex that is acquired for all callbacks.

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