net: ti: davinci_emac: stop using bus type mdio_bus_type

net: ti: davinci_emac: stop using bus type mdio_bus_type

This driver is the only user of mdio_bus_type outside phylib.
Using mdio_bus_type isn't strictly needed here, so use an alternative
approach. This will allow to make mdio_bus_type private to phylib
in a follow-up series.

Compile-tested only.

Note: Devices supported by this driver are OF-only, therefore the string
      comparison in match_first_device() isn't needed any longer.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/cc8e83aa-48c3-4497-b6ad-760a7f9e25dc@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>