net: phy: skip EEE advertisement write when autoneg is disabled

net: phy: skip EEE advertisement write when autoneg is disabled

genphy_c45_an_config_eee_aneg() writes the EEE advertisement to the
auto-negotiation device's MMD register space (MDIO_MMD_AN, register
MDIO_AN_EEE_ADV).  These registers are read by the link partner only
during auto-negotiation, so writing them while autoneg is disabled
cannot influence the link.  On some PHYs (e.g. Broadcom BCM54213PE)
the write nevertheless reaches the chip and disturbs the receive
datapath.

Concretely, running

    ethtool -s eth0 speed 100 duplex full autoneg off
    ethtool --set-eee eth0 eee off

leaves eth0 with TX working and RX completely silent on a
Raspberry Pi 4 / CM4 board (bcmgenet + BCM54213PE in rgmii-rxid).
Switching back to autoneg recovers the link.

Prior to commit f26a29a038ee ("net: phy: ensure that genphy_c45_an_config_eee_aneg() sees new value of phydev->eee_cfg.eee_enabled"),
the disable path was effectively a no-op because the helper read
the stale eee_cfg.eee_enabled, so the underlying PHY behavior never
surfaced.

Bisected on rpi-6.12.y between commits 83943264 (good) and
effcbc88 (bad) to f26a29a038ee.

Fixes: f26a29a038ee ("net: phy: ensure that genphy_c45_an_config_eee_aneg() sees new value of phydev->eee_cfg.eee_enabled")
Cc: stable@vger.kernel.org
Signed-off-by: Nerijus Bendžiūnas <nerijus.bendziunas@gmail.com>
Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Tested-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20260516150251.879680-1-nerijus.bendziunas@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>