net: dsa: mt7530: Separate C22 and C45 MDIO bus transactions

mt7530 does support C45, but its uses a mix of registering its MDIO
bus and providing its private MDIO bus to the DSA core, too. This makes
the change a bit more complex.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>