net: ethtool: phy: Convert the PHY_GET command to generic phy dump

net: ethtool: phy: Convert the PHY_GET command to generic phy dump

Now that we have an infrastructure in ethnl for perphy DUMPs, we can get
rid of the custom ->doit and ->dumpit to deal with PHY listing commands.

As most of the code was custom, this basically means re-writing how we
deal with PHY listing.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250502085242.248645-3-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>