ath9k: fix draining aggregation tid buffers

After a tx attempt, an A-MPDU subframe can still have fi->retries at 0
(if the retry count wasn't incremented due to powersave).
In that case it is still tracked as part of the block ack window, so
when draining the tid queue, its sequence number needs to be cleared
from the pending frame bitmap.

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