amd-xgbe-phy: PHY KX/KR mode differences

The PHY requires different settings for the Decision Feedback Analyzer
(DFE) when running in KX mode vs. KR mode. Update the code to change
these settings when changing modes in order to provide a more stable
link.

Additionally, adjust the 10GbE PQ skew default setting to a more sane
value.

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