net: pse-pd: Introduce attached_phydev to pse control

In preparation for reporting PSE events via ethtool notifications,
introduce an attached_phydev field in the pse_control structure.
This field stores the phy_device associated with the PSE PI,
ensuring that notifications are sent to the correct network
interface.

The attached_phydev pointer is directly tied to the PHY lifecycle. It
is set when the PHY is registered and cleared when the PHY is removed.
There is no need to use a refcount, as doing so could interfere with
the PHY removal process.

Signed-off-by: Kory Maincent (Dent Project) <kory.maincent@bootlin.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20250617-feature_poe_port_prio-v14-1-78a1a645e2ee@bootlin.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>