realtek: rtl930x: move serdes functions over to mdio bus

realtek: rtl930x: move serdes functions over to mdio bus

The migration of the RTL930x mdio/serdes access functions over to the
mdio bus is a little more complicated than for RTL83xx. There are several
places where the serdes is accessed directly. So do it in two steps. With
this first step:

- use the rtmdio prefix for the serdes reader/writer functions
- move the functions over to the bus (inside the ethernet driver)
- Adapt all callers.

This is not only a copy/paste but the serdes access will be hardened too.
For this:

- put a mutex around the read/write functions because we have only
  indirect register access through a mdio style bus.
- Verify input values to avoid data mess.

Tested-by: Bjørn Mork <[email protected]>
Tested-by: Jan Hoffmann <[email protected]>
Tested-by: Jonas Jelonek <[email protected]>
Signed-off-by: Markus Stockhausen <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19662
Signed-off-by: Robert Marko <[email protected]>