Merge branch 'net-phy-microchip-add-downshift-support-for-lan88xx'

Nicolai Buchwitz says:

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

Add standard ETHTOOL_PHY_DOWNSHIFT tunable support for the Microchip
LAN88xx PHY, following the same pattern used by Marvell and other PHY
drivers.

Ethernet cables with faulty or missing pairs (specifically C and D)
can successfully auto-negotiate 1000BASE-T but fail to establish a
stable link. The LAN88xx PHY supports automatic downshift to
100BASE-TX after a configurable number of failed attempts (2-5).

Patch 1 adds the get/set tunable implementation.
Patch 2 enables downshift by default with a count of 2. The setting is
stored in the driver's private data so that user changes via ethtool are
preserved across suspend/resume cycles.

Based on an earlier downstream implementation by Phil Elwell.

Tested on Raspberry Pi 3B+ (LAN7515/LAN88xx).
====================

Link: https://patch.msgid.link/20260401123848.696766-1-nb@tipi-net.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>