amd-xgbe-phy: Properly support the FEC auto-negotiation

Advertise and apply the Forward Error Correction capabilities of the
device based on the FEC ability of the device. Also, remove the use
of some hard coded values related to KR and FEC in preference of some
#defines.

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