mac80211: fix crash when using AP VLAN interfaces

Commit "mac80211: implement SMPS for AP" applies to AP_VLAN as well.
It assumes that sta->sdata->vif.bss_conf.bssid is present, which did not
get set for AP_VLAN.
Initialize it to sdata->vif.addr like for other interface types.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>