staging: gdm724x: gdm_mux: fix use-after-free on module unload

BugLink: http://bugs.launchpad.net/bugs/1692900

commit b58f45c8 upstream.

Make sure to deregister the USB driver before releasing the tty driver
to avoid use-after-free in the USB disconnect callback where the tty
devices are deregistered.

Fixes: 61e12104 ("staging: gdm7240: adding LTE USB driver")
Cc: Won Kang <wkang77@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

drivers/staging/gdm724x/gdm_mux.c

@@ -670,14 +670,14 @@ static int __init gdm_usb_mux_init(void)
 
 static void __exit gdm_usb_mux_exit(void)
 {
-	unregister_lte_tty_driver();
-
 	if (mux_rx_wq) {
 		flush_workqueue(mux_rx_wq);
 		destroy_workqueue(mux_rx_wq);
 	}
 
 	usb_deregister(&gdm_mux_driver);
+	unregister_lte_tty_driver();
+
 }
 
 module_init(gdm_usb_mux_init);