amd-xgbe-phy: Change auto-negotiation logic

The auto negotiation logic was geared to being the initiator of the
auto negotiation. This presented problems when auto negotiation was
initiated by the remote end. Change the auto negotiation logic to
make use of the auto negotiation event interrupt thus allowing the
auto negotiation state machine to function properly in either scenario.
This also removes the polling during auto-negotiation.

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