Fix exception in MPTTAdminForm

`exclude` requires a kwarg to filter on.

When I tried to view a model that used MPTTAdminForm I got an `AttributeError 'Folder' object has no attribute 'split'`. Looking at the history of that code it seems like a refactor accidentally introduced a bug.

Tested with Django 1.5.1.

Again sorry for closing the previous pull request. I'm doing a second request, which is a feature request, so I didn't want them to be tangled up together.