FIX: surf_ori is True for fixed fwd

I was working with some fixed orientation forward operators computed with MNE-C and noticed that `fwd['surf_ori']` was `False` for them, which made me think that there is something seriously wrong somewhere, as a fixed-orientation forward operator without surface orientation wouldn't make any sense. With this change, `fwd['surf_ori']` is always `True` for fixed forward operators, which makes sense to me but maybe I'm misunderstanding something..