net: usb: lan78xx: Integrate EEE support with phylink LPI API

Refactor Energy-Efficient Ethernet (EEE) support in the LAN78xx driver to
fully integrate with the phylink Low Power Idle (LPI) API. This includes:

- Replacing direct calls to `phy_ethtool_get_eee` and `phy_ethtool_set_eee`
  with `phylink_ethtool_get_eee` and `phylink_ethtool_set_eee`.
- Implementing `.mac_enable_tx_lpi` and `.mac_disable_tx_lpi` to control
  LPI transitions via phylink.
- Configuring `lpi_timer_default` to align with recommended values from
  LAN7800 documentation.
- ensure EEE is disabled on controller reset

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20250618122602.3156678-6-o.rempel@pengutronix.de


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