enetc: Set MDIO_CFG_HOLD to the recommended value of 2

This increases the MDIO hold time to 5 enet_clk cycles from the previous
value of 0. This is actually the out-of-reset value, that the driver was
previously overwriting with 0. Zero worked for the external MDIO, but
breaks communication with the internal MDIO buses on which the PCS of
ENETC SI's and Felix switch are found.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>