ath9k: remove struct ath_atx_ac

struct ath_atx_ac contains a list of active TIDs belonging to one WMM AC.
This patch changes the code to track active station TIDs in the txq directly.

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