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

Note that we no longer need to check for NULL as devm_fwnode_gpiod_get()
will return -ENOENT if GPIO is missing.
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lore.kernel.org/r/20191004231017.130290-7-dmitry.torokhov@gmail.comReviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>