enetc: Clean up serdes configuration

Decouple internal mdio bus creation from serdes
configuration, as a prerequisite to offloading
serdes configuration to a different module.
Group together mdio bus creation routines, cleanup.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>