net/mlx5: Add back multicast stats for uplink representor

The multicast counter got removed from uplink representor due to the
cited patch.

Fixes: 47c97e6b

 ("net/mlx5e: Fix multicast counter not up-to-date in "ip -s"")
Signed-off-by: Huy Nguyen <huyn@nvidia.com>
Reviewed-by: Daniel Jurgens <danielj@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>