ath9k: Don't trust TX status TID number when reporting airtime

As already noted a comment in ath_tx_complete_aggr(), the hardware will
occasionally send a TX status with the wrong tid number. If we trust the
value, airtime usage will be reported to the wrong AC, which can cause the
deficit on that AC to become very low, blocking subsequent attempts to
transmit.

To fix this, account airtime usage to the TID number from the original skb,
instead of the one in the hardware TX status report.

Reported-by: Miguel Catalan Cid <miguel.catalan@i2cat.net>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>