netdevsim: add pause frame stats

Add minimal ethtool interface for testing ethtool pause stats.

v2: add missing static on nsim_ethtool_ops

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>