wifi: rtl8xxxu: add cancel_work_sync() for c2hcmd_work

The workqueue might still be running, when the driver is stopped. To
avoid a use-after-free, call cancel_work_sync() in rtl8xxxu_stop().

Fixes: e542e66b ("rtl8xxxu: add bluetooth co-existence support for single antenna")
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/20240111163628.320697-2-martin.kaistra@linutronix.de