net: ethernet: renesas: rswitch: C45 only transactions

The rswitch MDIO bus driver only supports C45 transfers. Update the
function names to make this clear, pass the mmd as a parameter, and
register the accessors to the _c45 ops of the bus driver structure.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>