ath9k: Handle multiple keys while setting tx filters

The keycache index is used to abort transmission for given station
when it goes to sleep state. But the commit "ath9k_hw: Abort transmission
for sleeping station" is not handling multi-key station. Fix that.

Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>