amd-xgbe-phy: Use the proper auto-negotiation XNP registers

When receiving and processing extended next pages the base registers
were used instead of the XNP registers. Update the code to use the
device XNP and link partner XNP registers.

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