wifi: rtl8xxxu: support multiple interfaces in watchdog_callback()

Check first whether priv->vifs[0] exists and is of type STATION, then go
to priv->vifs[1]. Make sure to call refresh_rate_mask for both
interfaces.

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/20231222101442.626837-14-martin.kaistra@linutronix.de