gpio/gpio-ich: make ichx_gpio_check_available() return a pure boolean value

It is more readable for humans to use double-bang (!!) to convert the value
to pure boolean before it is returned.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>