amd-xgbe: Fixes for working with PHYs that support 2.5GbE

The driver has some missing functionality when operating in the mode that
supports 2.5GbE.  Fix the driver to fully recognize and support this speed.

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