nfp: add support restart of link auto-negotiation

Add support restart of link auto-negotiation.
This may be initiated using:

  # ethtool -r <intf>

Signed-off-by: Fei Qin <fei.qin@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>