serial: mctrl-gpio: drop usages of IS_ERR_OR_NULL

The function mctrl_gpio_init returns failure if the assignment to any
member of the gpio array results in an error pointer.  So there is no
need to check for such error values in the other functions.

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