ath9k: clean up and fix ath_tx_count_airtime

ath_tx_count_airtime is doing a lot of unnecessary work:

- Redundant station lookup
- Redundant rcu_read_lock/unlock
- Useless memcpy of bf->rates
- Useless NULL check of bf->bf_mpdu
- Redundant lookup of the skb tid

Additionally, it tries to look up the mac80211 queue index from the txq,
which fails if the frame was delivered via the power save queue.

This patch fixes all of these issues by passing down the right set of
pointers instead of doing extra work

Cc: stable@vger.kernel.org
Fixes: 63fefa05 ("ath9k: Introduce airtime fairness scheduling between stations")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>