realtek: pcs: drop setup_serdes

realtek: pcs: drop setup_serdes

Introducing the setup_serdes hook in the PCS driver was always intended
to be only a temporary solution. It was needed to be able to collect all
scattered code in the PCS driver first. Later, refactoring can be done
to bring everything into a proper shape.

Now we've reached the point where the setup of all variants has been
refactored in such a way that all share the same high-level sequence.
This is fully orchestrated in pcs_config now. Thus, the setup_serdes hook
is no longer needed. Move the last piece out of it into another SerDes
op hook, then drop setup_serdes.

Link: https://github.com/openwrt/openwrt/pull/23861
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>