net: phy: remove modalias-based mdio bus matching

Last user dsa_loop has been migrated away from modalias-based matching,
so we can remove this feature now. It was the only user of MDIO_NAME_SIZE,
so remove also this constant.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/ce1c6df0-4785-4b28-8322-32dc6bceea18@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>