Add minimal ethtool interface for testing ethtool pause stats.

v2: add missing static on nsim_ethtool_ops
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Reviewed-by: NSaeed Mahameed <saeedm@nvidia.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>