gpio-ich: Fix value returned by ichx_gpio_request

Per Documentation/gpio.txt, gpio_request callbacks should return 0 on
success or a negative value on error. So it is not clear what was
meant by letting ichx_gpio_request return 1 in some cases, nor how a
caller would interpret it.

Align the code with the comment above it and consider pins as
available by default.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>