I think that its placement makes it appear as if you're talking about the restaurant, not the soup. The comma, however, makes sense to me. Don't take my word alone for it, though. I'm a comma-sprinkling fool. :)
(We say "bowl" of soup, not "plate", at least in AE. I don't think I've heard "plate of soup" before, either from an AE or BE speaker.)
How about:
"I just went to the campus restaurant and had a bowl of soup, which was good."
I disagree. The comma makes it clear that you are not referring to the restaurant, compare it with the following;
"I just had a plate of soup at the campus restaurant which was good."
This indicates that the other restaurants on campus are not good - I ate in the one which was good.
In fact, the way the original sentence is constructed implies that the whole experience of having a bowl of soup at the restaurant was good, as does;
"
I just went to the campus restaurant and had a bowl of soup, which was good."
If you want to say that the soup was good or tasty say;
"I had a really good tasty bowl of soup in the campus restaurant."
I do agree that a bowl is much better than a plate for serving soup.:)