mlx4: add pause frame stats

Check if the pause stats are reported by HW by checking the bitmap.
Calculation is based on the order of strings in main_strings from
ethtool -S. Hopefully the semantics of these stats match the standard..

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