RTL930x devices need the USXGMII mode. This is a final leftover
from the 6.6 conversion.
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/16457
Signed-off-by: Robert Marko <robimarko@gmail.com>