net: aquantia: Add renegotiate ethtool operation support

Adds ethtool -r|--negotiate operation support. It triggers special
control bit on FW interface causing FW to restart link negotiation.

Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: Anton Mikaev <amikaev@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>