net: fec_mxc: fix probe of MDIO bus for DM_MDIO

net: fec_mxc: fix probe of MDIO bus for DM_MDIO

When initializing the MDIO bus the MDC signal needs to be configured.
Otherwise the communication over the bus may fail. Add the call to
fec_mii_setspeed to the DM_MDIO probe handler.

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>