realtek: pcs: rtl930x: rework SerDes mode setting

realtek: pcs: rtl930x: rework SerDes mode setting

Rework the SerDes mode setting to have a better logical structure,
similar to how RTL931x currently does it. Mode setting is grouped into a
MAC setup and IP core setup. Most modes just need to trigger the MAC
setup and then they work, otherwise some complex sequence is needed.

To achieve that, rename several functions and group their content
differently. While at it, rename some constants to use the RTPCS_ prefix.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21565
Signed-off-by: Robert Marko <robimarko@gmail.com>