gpiolib: acpi: Use named item for enum gpiod_flags variable

Use named item instead of plain integer for enum gpiod_flags
to make it clear that even 0 has its own meaning.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>