ath9k: improve powersave filter handling

For non-aggregated frames, ath9k was leaving handling of powersave
filtered packets to mac80211. This can be too slow if the intermediate
queue is already filled with packets and mac80211 does not immediately
send a new packet via drv_tx().

Improve response time with filtered frames by triggering clearing the
powersave filter internally.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>