net: phy: fixed_phy: switch to using fwnode_gpiod_get_index

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.

Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>