realtek: pcs: rtl931x: set SerDes to off early

realtek: pcs: rtl931x: set SerDes to off early

Set the SerDes mode to OFF early in the setup process to have a clear
starting point. This was part of the media handling before as a leftover
from the SDK code import. In the SDK, this function didn't only care
about applying some media settings but was also some kind of mode
setting for fiber modes.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22786
Signed-off-by: Robert Marko <robimarko@gmail.com>