gpio: gpio-mxc: gpio_set_wake_irq() use proper return values

Errors from enable_irq_wake() in gpio_set_wake_irq() were silently ignored.
Thus led to the problem that gpio_set_wake_irq() always returned
successfully, even if enable_irq_wake() returned an error.

Signed-off-by: Philipp Rosenberger <p.rosenberger@linutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>