mac80211: flush keys for AP mode on ieee80211_do_stop

Userspace can add keys to an AP mode interface before start_ap has been
called. If there have been no calls to start_ap/stop_ap in the mean
time, the keys will still be around when the interface is brought down.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
[adjust comments, fix AP_VLAN case]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>