serial: mctrl-gpio: don't check for struct mctrl_gpios * to be invalid

Drivers using mctrl-gpio must not pass invalid values for struct
mctrl_gpios *. All drivers were fixed in this regard and so some checks
can go away or be simplified.

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