USB: g_serial: fix tty cleanup on unload

Call put_tty_driver() in cleanup function, to fix Oops when trying to open
gadget serial char device after module unload.

Signed-off-by: Jon Povey <jon.povey@racelogic.co.uk>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/u_serial.c

@@ -1184,6 +1184,7 @@ void gserial_cleanup(void)
 	n_ports = 0;
 
 	tty_unregister_driver(gs_tty_driver);
+	put_tty_driver(gs_tty_driver);
 	gs_tty_driver = NULL;
 
 	pr_debug("%s: cleaned up ttyGS* support\n", __func__);