net/mlx5e: Make mlx5e_dcbnl_ops static

Fix sparse warning:
drivers/net/ethernet/mellanox/mlx5/core/en_dcbnl.c:988:29:
error: symbol 'mlx5e_dcbnl_ops' was not declared. Should it be static?

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>