Fix potential tty-kref leak in stop_rx path.

Cc: stable@vger.kernel.org
Signed-off-by: NJohan Hovold <jhovold@gmail.com>
Tested-by: NStephen Warren <swarren@nvidia.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>