ethtool: Allow passing a PHY index for phy-targetting commands

With the introduction of PHY topology and the ability to list PHYs, we
can now target some netlink commands to specific PHYs. This is done by
passing a PHY index as a request parameter in the netlink GET command.

This is useful for PSE-PD, PLCA and Cable-testing operations when
multiple PHYs are on the link (e.g. when a PHY is used as an SFP
upstream controller, and when there's another PHY within the SFP
module).

Introduce a new, generic, option "--phy N" that can be used in
conjunction with PHY-targetting commands to pass the PHY index for the
targetted PHY.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>