net: ethtool: Introduce per-PHY DUMP operations

net: ethtool: Introduce per-PHY DUMP operations

ethnl commands that target a phy_device need a DUMP implementation that
will fill the reply for every PHY behind a netdev. We therefore need to
iterate over the dev->topo to list them.

When multiple PHYs are behind the same netdev, it's also useful to
perform DUMP with a filter on a given netdev, to get the capability of
every PHY.

Implement dedicated genl ->start(), ->dumpit() and ->done() operations
for PHY-targetting command, allowing filtered dumps and using a dump
context that keep track of the PHY iteration for multi-message dump.

PSE-PD and PLCA are converted to this new set of ops along the way.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250502085242.248645-2-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>