wifi: rtl8xxxu: Allow creating interface in AP mode

Use the sequence from the vendor driver for setting up the beacon
related registers.
Also set the MAC address register here, in case the MAC address for the
new interface should be different from what was set in
rtl8xxxu_init_device(). This happens for example with the hostapd config
option "bssid".

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://lore.kernel.org/r/20230428150833.218605-7-martin.kaistra@linutronix.de