serial: imx: reorder functions and simplify a bit

Now that imx_mctrl_check is implemented below imx_get_mctrl the former
can call the latter directly instead of via sport->port.ops->get_mctrl.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>