Fix apparent landbset insertion/removal logic issue

I won't pretend to understand the full logic here at present, but:

1. While investigating an Octavia bug I noticed the parameter count when calling the validation is wrong, which can't be good no matter what.
2. Logic of the check depending on a supplied 'device' param does not make sense to me. But if there is something I'm missing we can just drop the 2nd commit here.

Still unclear: Why is there a device parameter at all? Did it have a purpose before? At this point I guess we keep it for compatibility with existing code that uses it (nova, for instance).