realtek: RTL931x: disable USXGMII SerDes setup

realtek: RTL931x: disable USXGMII SerDes setup

The first RTL931x devices make their way into OpenWrt. Their copper
ports are driven by different interfaces modes like 10G_QXGMII or
Realtek proprietary XSGMII. The DSA driver has no proper handling
for theses modes implemented yet. So a lot is auto-mapped to USXGMII
internally. As soon as the SerDes setup activates this (wrong) mode
the PHY connectivity breaks.

Disable this mode for now and rely on the proper U-Boot setup.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/20292
Signed-off-by: Robert Marko <robimarko@gmail.com>