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

[ Upstream commit b58f45c8 ]

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: stable <stable@vger.kernel.org>     # 3.12
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: Sasha Levin <alexander.levin@verizon.com>