realtek: rtl931x: align SerDes access with other targets

realtek: rtl931x: align SerDes access with other targets

While converting the RTL931x SerDes code to the new frontend
access methods, the target specific workarounds where left in
place. The old functions were kept and the phy/sds mapping
was unchanged too. It is time to clean this up

- drop the old functions
- reuse the existing read/write logic
- harden the new functions

For now keep the function naming rtmdio_...__new() as is. This
will be changed in a future commit.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19973
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>