net/mlx5e: Cleanup of dcbnl related fields

Remove unused netdev_registered_init/remove in en.h
Return ENOSUPPORT if the check MLX5_DSCP_SUPPORTED fails.
Remove extra white space

Fixes: 2a5e7a13

 ("net/mlx5e: Add dcbnl dscp to priority support")
Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Cc: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>