net/mlx5e: Move dl_port to struct mlx5e_dev

No need to have dl_port which is tightly coupled with mlx5e code
in mlx5 core code. Move it to struct mlx5e_dev and loose
mlx5e_devlink_get_dl_port() helper.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>