wifi: rtl8xxxu: support multiple interfaces in update_beacon_work_callback()

As we only want to support AP mode/sending beacons on port 0, it is
enough to replace priv->vif with priv->vifs[0].
Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20231222101442.626837-12-martin.kaistra@linutronix.de

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c

@@ -5712,7 +5712,7 @@ static void rtl8xxxu_update_beacon_work_callback(struct work_struct *work)
 	struct rtl8xxxu_priv *priv =
 		container_of(work, struct rtl8xxxu_priv, update_beacon_work);
 	struct ieee80211_hw *hw = priv->hw;
-	struct ieee80211_vif *vif = priv->vif;
+	struct ieee80211_vif *vif = priv->vifs[0];
 
 	if (!vif) {
 		WARN_ONCE(true, "no vif to update beacon\n");