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

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c

@@ -7480,6 +7480,7 @@ static void rtl8xxxu_stop(struct ieee80211_hw *hw)
 	if (priv->usb_interrupts)
 		rtl8xxxu_write32(priv, REG_USB_HIMR, 0);
 
+	cancel_work_sync(&priv->c2hcmd_work);
 	cancel_delayed_work_sync(&priv->ra_watchdog);
 
 	rtl8xxxu_free_rx_resources(priv);