amd-xgbe: Set the MII control width for the MAC interface

When running in SGMII mode at speeds below 1000Mbps, the auto-negotition
control register must set the MII control width for the MAC interface
to be 8-bits wide.  By default the width is 4-bits.

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