RTL930x uses the IP-driven mode setup for Base-X links and the 1G
analog configuration for SGMII and 1000BASE-X.
Route 100BASE-X through the same IP-driven and 1G analog setup paths.
Keep the mode unavailable until all RTL93xx support is in place.
Link: https://github.com/openwrt/openwrt/pull/25378
Signed-off-by: Jonas Jelonek <jonas@jonasjelonek.de>