realtek: pcs: rtl931x: move SerDes config out of setup

realtek: pcs: rtl931x: move SerDes config out of setup

Slim the rtpcs_931x_setup_serdes entrypoint by moving the SerDes
configuration sequences for the different SerDes modes into a dedicated
function called rtpcs_931x_sds_config_mode. They form a logical block
similar to what the SDK does and can be taken out to follow the
'divide & conquer' principle.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21184
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>