ath9k: don't use BAW tracking on PS responses for non-AMPDU packets

When .release_buffered_frames was implemented, only A-MPDU packets were
buffered internally. Now that this has changed, the BUF_AMPDU flag needs
to be checked before calling ath_tx_addto_baw

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>