gpio/gpio-ich: fix ichx_gpio_check_available() return what callers expect

ichx_gpio_check_available() returns either 0 or -ENXIO depending on whether
the given GPIO is available or not. However, callers of this function treat
the return value as boolean:

	...
	if (!ichx_gpio_check_available(gpio, nr))
		return -ENXIO;

which erroneusly fails when the GPIO is available and not vice versa.

Fix this by making the function return boolean as expected by the callers.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>