gpio: propagate of_parse_phandle_with_args errors

Make of_get_named_gpio_flags propagate any error it receives from
of_parse_phandle_with_args instead of inconditionally returning -EINVAL.

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