serial: mxs-auart: properly handle mctrl_gpio failing

If mctrl_gpio_init returns an error code this value should be forwarded and
the driver must not simply ignore this failure.

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