amd-xgbe: Be sure to set MDIO modes on device (re)start

The MDIO register mode is set when the device is probed. But when the
device is brought down and then back up, the MDIO register mode has been
reset.  Be sure to reset the mode during device startup and only change
the mode of the address specified.

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