gpiolib: of: remove gpio_to_desc() usage

As demonstrated by commit
390d82e3
"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.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>