net: phy: mediatek-ge: disable EEE on the MT7530 PHY

The MT7530 internal GE PHY advertises EEE by hardware default, but its
EEE support is defective: with EEE advertised, some link partners fail
to establish a stable link. On a 2-pair (4-wire) cable where both ends
advertise gigabit, 1000BASE-T training cannot succeed, and instead of
falling back to 100 Mbps the port loops, so no link or DHCP lease is
ever obtained. MediaTek confirms the hardware is the root cause (Landen
Chao, 2021): "EEE of the 10-year-old MT7530 internal gephy has many IOT
problems, so it is recommended to disable its EEE."

mtk_gephy_config_init() used to clear the EEE advertisement early, but
commit af3b4b0e59de ("net: phy: mediatek-ge: do not disable EEE
advertisement") removed that on the rationale that the DSA subdriver
already performs an early disable. That holds for MT7531, whose
mt7531_setup() clears MDIO_AN_EEE_ADV on each switch PHY, but not for
the MT7530 PHY: neither the MT7621 integrated switch nor the dedicated
MT7530 IC ever had such a loop, so removing it left those boards
without any working early EEE disable and the link flapping came back.

Since the broken hardware is the PHY, fix it in the PHY driver so it
covers all users of this PHY, integrated in a switch or standalone:

  - clear MDIO_AN_EEE_ADV in probe(), as early as possible, before
    anything can negotiate EEE with the link partner;
  - clear it again in config_init() and call phy_disable_eee() there.
    config_init() is what phy_init_hw() replays after a PHY reset, when
    the register is back at its EEE-advertising hardware default, and
    it runs after of_set_phy_eee_broken() in phy_probe(), so the
    eee_disabled_modes mask survives and neither phylib nor userspace
    can re-enable EEE. dp83867 disables broken EEE from config_init()
    the same way.

Auto-negotiation then falls back to a stable 100 Mbps link instead of
looping at gigabit. Tested on ASUS RT-AX53U (MT7621): with a 2-pair
cable on the WAN port, a single clean 100 Mbps link comes up and a
DHCP lease is obtained, where the unpatched driver loops.

Fixes: af3b4b0e59de ("net: phy: mediatek-ge: do not disable EEE advertisement")
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Vladislav Karmanov <vladislav.karmanov.dev@gmail.com>
Link: https://patch.msgid.link/20260908145213.3976508-1-vladislav.karmanov.dev@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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