gpiolib: ACPI: remove gpio_to_desc() usage

gpio_to_desc() must die. Replace one of its usage by the
newly-introduced gpiochip_get_desc() function.

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