net/mlx5: Don't skip vport check

Users of mlx5_eswitch_get_vport() are required to check return value
prior to passing mlx5_vport further. Fix all the places to do not skip
that check.

Reviewed-by: Eli Cohen <elic@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>