net/mlx5e: Add txq to sq stats mapping

mlx5 currently maps txqs to an sq via priv->txq2sq. It is useful to map
txqs to sq_stats, as well, for direct access to stats.

Add priv->txq2sq_stats and insert mappings. The mappings will be used
next to tabulate stats information.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>