ath9k: wake hardware during AMPDU TX actions

AMDPDU actions poke hardware for TX operation, as such
we want to turn hardware on for these actions. AMDPU RX operations
do not require hardware on as nothing is done in hardware for
those actions. Without this we cannot guarantee hardware has
been programmed correctly for each AMPDU TX action.

Cc: stable@kernel.org
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>