net: fec_mxc: fix remove with DM_MDIO

net: fec_mxc: fix remove with DM_MDIO

If DM_MDIO is used and the FEC device is removed the mdio API
must not be used to remove the bus structure. Store pointer the
the udevice for MDIO bus created by dm_fec_bind_mdio and use DM
functions to cleanup the device in fecmxc_remove.

Fixes: 3b8f99a3e762 ("net: fec: add support for DM_MDIO")
Signed-off-by: Markus Niebel <Markus.Niebel@ew.tq-group.com>
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Alexander Feilke <alexander.feilke@ew.tq-group.com>