wcn36xx: Advertise beacon filtering support in bmps

In bmps mode, beacons are filtered, and firmware is in charge
of monitoring the beacons and report changes or loss.

mac80211 must be advertised about such change to prevent it's
internal timer based beacon monitor to report beacon loss.

Fix that by setting/clearing the IEEE80211_VIF_BEACON_FILTER
vif flag on bmps entry/exit.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1592471863-31402-2-git-send-email-loic.poulain@linaro.org