net: dsa: mv88e6xxx: Separate C22 and C45 transactions

The global2 SMI MDIO bus driver can perform both C22 and C45
transfers. Create separate functions for each and register the C45
versions using the new API calls where appropriate. Update the SERDES
code to make use of these new accessors.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>