net: phy: smsc: Fix link failure in forced mode with Auto-MDIX

Force a fixed MDI-X mode when auto-negotiation is disabled to prevent
link instability.

When forcing the link speed and duplex on a LAN9500 PHY (e.g., with
`ethtool -s eth0 autoneg off ...`) while leaving MDI-X control in auto
mode, the PHY fails to establish a stable link. This occurs because the
PHY's Auto-MDIX algorithm is not designed to operate when
auto-negotiation is disabled. In this state, the PHY continuously
toggles the TX/RX signal pairs, which prevents the link partner from
synchronizing.

This patch resolves the issue by detecting when auto-negotiation is
disabled. If the MDI-X control mode is set to 'auto', the driver now
forces a specific, stable mode (ETH_TP_MDI) to prevent the pair
toggling. This choice of a fixed MDI mode mirrors the behavior the
hardware would exhibit if the AUTOMDIX_EN strap were configured for a
fixed MDI connection.

Fixes: 05b35e7eb9a1 ("smsc95xx: add phylib support")
Signed-off-by: Oleksij Rempel <[email protected]>
Cc: Andre Edich <[email protected]>
Reviewed-by: Andrew Lunn <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

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