arm/imx/iomux-v1: check for invalid modes in mxc_gpio_mode

mxc_gpio_mode checks for invalid pins and so it returns zero for
success, -EINVAL for invalid pins.

While at it, remove definitions of GPIO_PORT_MAX removed as they are
unused now.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>