wifi: rtl8xxxu: enable channel switch support

The CSA countdown in the beacon frames, which are sent out by firmware,
needs to get updated by the driver. To achieve this, convert
update_beacon_work to delayed_work and schedule it with the beacon
interval in case CSA is active and the countdown is not complete.

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/20240111163628.320697-3-martin.kaistra@linutronix.de