RDMA/mlx5: Add optional counter support in get_hw_stats callback

When get_hw_stats is called, query and return the optional counter
statistic as well.

Link: https://lore.kernel.org/r/20211008122439.166063-14-markzhang@nvidia.com


Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>