net/mlx5: Use one completion vector if eth is disabled

If eth is disabled by devlink, use only a single completion vector to
have minimum performance of all users of completion vectors. This also
affects Infiniband performance.

The rest of the vectors can be used by other consumers on a first come
first served basis.

mlx5_vdpa will make use of this to allocate dedicated vectors for its
own use.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>