ath9k: fix tracking of enabled AP beacons

sc->nbcnvifs tracks assigned beacon slots, not enabled beacons.
Therefore, it cannot be used to decide if cur_conf->enable_beacon (bool)
should be updated, or if beacons have been enabled already.
With the current code (depending on the order of calls), beacons often
do not get enabled in an AP+STA setup.
To fix tracking of enabled beacons, convert cur_conf->enable_beacon to a
bitmask of enabled beacon slots.

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