regulator: slg51000: switch to using fwnode_gpiod_get_index

regulator: slg51000: switch to using fwnode_gpiod_get_index

devm_gpiod_get_from_of_node() is being retired in favor of
[devm_]fwnode_gpiod_get_index(), that behaves similar to
devm_gpiod_get_index(), but can work with arbitrary firmware node. It
will also be able to support secondary software nodes.

Let's switch this driver over.

Note that now that we have a good non-devm API for getting GPIO from
arbitrary firmware node, there is no reason to use devm API here as
regulator core takes care of managing lifetime of "enable" GPIO and we
were immediately detaching requested GPIO from devm anyway.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lore.kernel.org/r/20191004231017.130290-3-dmitry.torokhov@gmail.com
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>