wifi: rtl8xxxu: support multiple interface in start_ap()

Call set_bssid() with the correct port_num now.

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/20231222101442.626837-17-martin.kaistra@linutronix.de