net: phy: microchip: enable downshift by default on LAN88xx

net: phy: microchip: enable downshift by default on LAN88xx

Enable auto-downshift from 1000BASE-T to 100BASE-TX after 2 failed
auto-negotiation attempts by default. This ensures that links with
faulty or missing cable pairs (C and D) fall back to 100Mbps without
requiring userspace configuration.

The downshift count is stored in the driver's private data and applied
in config_init, so user changes via ethtool are preserved across
suspend/resume cycles.

Users can override or disable downshift at runtime:

  ethtool --set-phy-tunable eth0 downshift off

Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20260401123848.696766-3-nb@tipi-net.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>