ath9k: fix block-ack window tracking issues

Ensure that a buffer gets tracked as part of the block-ack window as
soon as it's dequeued from the tid for the first time. Ensure that
double calls to ath_tx_addto_baw (e.g. on retransmission) don't cause
any issues.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>