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: ccbde4b128ef ("char: xillybus: Don't destroy workqueue from work item running on it")
Cc: stable <[e-mail]>
Signed-off-by: Eli Billauer <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/char/xillybus/xillyusb.c

···
 	 * 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;
 
···
 
 static void __exit xillyusb_exit(void)
 {
-	destroy_workqueue(wakeup_wq);
-
 	usb_deregister(&xillyusb_driver);
+
+	destroy_workqueue(wakeup_wq);
 }
 
 module_init(xillyusb_init);