amd-xgbe: Set the MDIO mode for 10000Base-T configuration

Currently the MDIO mode is set to none for the 10000Base-T, which is
incorrect.  The MDIO mode should for this configuration should be
clause 45.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>