serial: clps711x: fail if mctrl_gpio_init fails

mctrl_gpio_init is fully aware of being optional. If it returns an error
code this indicates a real error that must not be ignored.

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