realtek: dsa: do not open code PHY access

realtek: dsa: do not open code PHY access

The DSA has a link to the MDIO bus and already uses the read/write functions
that are provided. In parallel the dsa_switch_ops structure provides an
interface for phy_read and phy_write. These are still open-coded and sadly
circumvent the bus. Simplify the implementation and avoid inconsistencies by
reusing the existing bus infrastructure.

Additionally, remove two unused MMD header definitions as a quick win.

Reported-by: Jan Hoffmann <jan@3e8.eu>
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19548
Signed-off-by: Robert Marko <robimarko@gmail.com>