serial: tegra: fix tty-kref leak

commit cfd29aa0e81b791985e8428e6507e80e074e6730 upstream.

Fix potential tty-kref leak in stop_rx path.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>