net: phy: microchip: add downshift tunable support for LAN88xx

net: phy: microchip: add downshift tunable support for LAN88xx

Implement the standard ETHTOOL_PHY_DOWNSHIFT tunable for the LAN88xx
PHY. This allows runtime configuration of the auto-downshift feature
via ethtool:

  ethtool --set-phy-tunable eth0 downshift on count 3

The LAN88xx PHY supports downshifting from 1000BASE-T to 100BASE-TX
after 2-5 failed auto-negotiation attempts. Valid count values are
2, 3, 4 and 5.

This is based on an earlier downstream implementation by Phil Elwell.

Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20260401123848.696766-2-nb@tipi-net.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>