wifi: rtl8xxxu: support setting bssid register for multiple interfaces

To prepare for concurrent mode, enhance rtl8xxxu_set_bssid() to write the
BSSID of the respective interface to REG_BSSID or REG_BSSID1.

Like done with rtl8xxxu_set_mac(), call rtl8xxxu_set_bssid() with
port_num = 0, until the callers also support multiple 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/20231222101442.626837-10-martin.kaistra@linutronix.de