ath9k: fix tx aggregation flush on AR9003

Completing aggregate frames can lead to new buffers being pushed into
the tid queues due to software retransmission.
When the tx queues are being drained, all pending aggregates must be
completed before the tid queues get drained, otherwise buffers might be
leaked.

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