realtek: 6.6: change to current dsa structures

realtek: 6.6: change to current dsa structures

The DSA framework has changed a bit since 6.1, lets adapt to match.
Currently there is no one-patch-fits-all solution to directly fix
all errors up to 6.6. So cover the final differences with this
second patch.

Most notable upstream changes are:
  - a88dd7538461 ("net: dsa: remove legacy_pre_march2020 detection")
  - 53d04b981110 ("net: dsa: remove phylink_validate() method")

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
[Minor checkpatch.pl cleanups]
Signed-off-by: Sander Vanheule <sander@svanheule.net>