net/mlx5e: Pass mdev to mlx5e_devlink_port_register()

Instead of accessing priv->mdev, pass mdev pointer to
mlx5e_devlink_port_register() and access it directly.

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