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