serial: imx: disable UCR4_OREN on shutdown

UCR4_OREN is (depending on the configuration) enabled in startup,
but is never disabled. Fix this by disabling it in shutdown.

Reported-by: Nandor Han <nandor.han@ge.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>