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>