amd-xgbe: Fix flow control setting logic

The flow control negotiation logic is flawed and does not properly
advertise and process auto-negotiation of the flow control settings.
Update the flow control support to properly set the flow control
auto-negotiation settings and process the results approrpriately.

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