amd-xgbe: Add support for SFP+ modules

Add support for recognizing and using SFP+ modules directly. This includes
using the I2C support to read and interpret the information returned from
an SFP+ module and configuring things properly.

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