staging: frontier: use after free in disconnect()

usb_alphatrack_delete() frees "dev" so we can't use it on that path.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/frontier/alphatrack.c

@@ -827,11 +827,11 @@ static void usb_alphatrack_disconnect(struct usb_interface *intf)
 		mutex_unlock(&dev->mtx);
 		usb_alphatrack_delete(dev);
 	} else {
+		atomic_set(&dev->writes_pending, 0);
 		dev->intf = NULL;
 		mutex_unlock(&dev->mtx);
 	}
 
-	atomic_set(&dev->writes_pending, 0);
 	mutex_unlock(&disconnect_mutex);
 
 	dev_info(&intf->dev, "Alphatrack Surface #%d now disconnected\n",