mlx5: add pause frame stats

Plumb through all the indirection and copy some code from
ethtool -S. The names of the group indicate that these are
the stats we are after (and Saeed confirms it).

v3:
 - fix build in mlx5_rep
v2:
 - drop the ethool helper and call stats directly
 - don't pass 0 as initialized to in buffer
 - use local buffer

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