net/mlx5: Enable SD feature

Have an actual mlx5_sd instance in the core device, and fix the getter
accordingly. This allows SD stuff to flow, the feature becomes supported
only here.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>