net: bcmgenet: support restarting auto-negotiation

Hook a nway_reset ethtool callback to allow restarting the
auto-negotiation process when asked to. We defer to the PHY library call
to do the heavy lifting.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>