char: xillybus: Refine workqueue handling

As the wakeup work item now runs on a separate workqueue, it needs to be
flushed separately along with flushing the device's workqueue.

Also, move the destroy_workqueue() call to the end of the exit method,
so that deinitialization is done in the opposite order of
initialization.

Fixes: ccbde4b ("char: xillybus: Don't destroy workqueue from work item running on it")
Cc: stable <stable@kernel.org>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Link: https://lore.kernel.org/r/20240816070200.50695-1-eli.billauer@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/xillybus/xillyusb.c

@@ -2093,9 +2093,11 @@ static int xillyusb_discovery(struct usb_interface *interface)
 	 * just after responding with the IDT, there is no reason for any
 	 * work item to be running now. To be sure that xdev->channels
 	 * is updated on anything that might run in parallel, flush the
-	 * workqueue, which rarely does anything.
+	 * device's workqueue and the wakeup work item. This rarely
+	 * does anything.
 	 */
 	flush_workqueue(xdev->workq);
+	flush_work(&xdev->wakeup_workitem);
 
 	xdev->num_channels = num_channels;
 
@@ -2274,9 +2276,9 @@ static int __init xillyusb_init(void)
 
 static void __exit xillyusb_exit(void)
 {
-	destroy_workqueue(wakeup_wq);
-
 	usb_deregister(&xillyusb_driver);
+
+	destroy_workqueue(wakeup_wq);
 }
 
 module_init(xillyusb_init);