mac80211: clear SSID when stopping AP

When AP interface is stopped ssid_len in the BSS configuration
isn't cleared which can confuse drivers when switching modes.
Set the length to zero when stopping the AP interface.
Signed-off-by: Marek Puzyniak <marek.puzyniak@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/cfg.c

@@ -1052,6 +1052,7 @@ static int ieee80211_stop_ap(struct wiphy *wiphy, struct net_device *dev)
 	ieee80211_free_keys(sdata);
 
 	sdata->vif.bss_conf.enable_beacon = false;
+	sdata->vif.bss_conf.ssid_len = 0;
 	clear_bit(SDATA_STATE_OFFCHANNEL_BEACON_STOPPED, &sdata->state);
 	ieee80211_bss_info_change_notify(sdata, BSS_CHANGED_BEACON_ENABLED);