gpio: mxc: Do not hard code return value

Silences the following warning:
why not propagate 'port->irq' from platform_get_irq() instead of (-22)?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>