gpio: gpio-mxc: Fix: higher 16 GPIOs usable as wake source

In the function gpio_set_wake_irq(), port->irq_high is only checked for
zero. As platform_get_irq() returns a value less then zero if no interrupt
was found, any gpio >= 16 was handled like an irq_high interrupt was
available. On iMX27 for example no high interrupt is available. This lead
to the problem that only some gpios (the lower 16) were useable as wake
sources.

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