net: usb: lan78xx: Remove KSZ9031 PHY fixup

net: usb: lan78xx: Remove KSZ9031 PHY fixup

Remove the KSZ9031RNX PHY fixup from the lan78xx driver. The fixup applied
specific RGMII pad skew configurations globally, but these settings violate the
RGMII specification and cause more harm than benefit.

Key issues with the fixup:
1. **Non-Compliant Timing**: The fixup's delay settings fall outside the RGMII
   specification requirements of 1.5 ns to 2.0 ns:
   - RX Path: Total delay of **2.16 ns** (PHY internal delay of 1.2 ns + 0.96
     ns skew).
   - TX Path: Total delay of **0.96 ns**, significantly below the RGMII minimum
     of 1.5 ns.

2. **Redundant or Incorrect Configurations**:
   - The RGMII skew registers written by the fixup do not meaningfully alter
     the PHY's default behavior and fail to account for its internal delays.
   - The TX_DATA pad skew was not configured, relying on power-on defaults
     that are insufficient for RGMII compliance.

3. **Micrel Driver Support**: By setting `PHY_INTERFACE_MODE_RGMII_ID`, the
   Micrel driver can calculate and assign appropriate skew values for the
   KSZ9031 PHY.  This ensures better timing configurations without relying on
   external fixups.

4. **System Interference**: The fixup applied globally, reconfiguring all
   KSZ9031 PHYs in the system, even those unrelated to the LAN78xx adapter.
   This could lead to unintended and harmful behavior on unrelated interfaces.

While the fixup is removed, a better mechanism is still needed to dynamically
determine the optimal combination of PHY and MAC delays to fully meet RGMII
requirements without relying on Device Tree or global fixups. This would allow
for robust operation across different hardware configurations.

The Micrel driver is capable of using the interface mode value to calculate and
apply better skew values, providing a configuration much closer to the RGMII
specification than the fixup. Removing the fixup ensures better default
behavior and prevents harm to other system interfaces.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241204084142.1152696-3-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>