realtek: mdio: pull bus/ctrl dependency apart

realtek: mdio: pull bus/ctrl dependency apart

Until now the central control structure is allocated via a call to
devm_mdiobus_alloc_size(). This will not be possible any longer when
multiple busses will be implemented in a future commit.

Relax that as follows:

- Define a new private "channel" structure for a mdio bus
- Allocate the central control structure with a dedicated alloc()
- Allocate only the channel structure during bus setup
- Link the channel to the central structure via chan->ctrl

Reorganize the probing function so that it becomes clearer that
the control structure is setup first and afterwards the bus is
registered.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/22604
Signed-off-by: Robert Marko <robimarko@gmail.com>