leds: lgm-sso: switch to using devm_fwnode_gpiod_get()

devm_fwnode_get_gpiod_from_child() is going away as the name is too
unwieldy, let's switch to using the new devm_fwnode_gpiod_get().

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>