iwlwifi: mvm: don't ask for beacons when AP vif and no assoc sta

When in AP mode, we need beacons from other APs for HT protection.
However, when there's no any associated station we will not do
any Tx and thus don't really need beacons. On the other hand, these
beacons will cause a lot of unnecessary wakeups which increase our
power consumption. Handle this by asking FW to pass beacons only when
there's at least one associated station.

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>