wifi: rtl8xxxu: Perform update_beacon_work when beaconing is enabled

In STA+AP concurrent mode, performing a scan operation on one vif
temporarily stops beacons on the other. When the scan is completed,
beacons are enabled again with BSS_CHANGED_BEACON_ENABLED.

We can observe that no beacons are being sent when just
rtl8xxxu_start_tx_beacon() is being called.

Thus, also perform update_beacon_work in order to restore beaconing.

Fixes: cde8848c ("wifi: rtl8xxxu: Add beacon functions")
Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240930084955.455241-1-martin.kaistra@linutronix.de