wifi: rtl8xxxu: Add start_ap() callback

This gets called at the start of AP mode operation. Set bssid, beacon
interval and send a connect report to the HW.

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/20230428150833.218605-2-martin.kaistra@linutronix.de