_linkimagefield_format_set_attribute prevents imagefield_tokens from working

Added linkimagefield to imagefield_tokens list of supported modules (in the file), but oddly wasn't working. Tracked it down to _linkimagefield_format_set_attribute checking if custum values was added, preventing the imagefield_tokens from doing it's jobs.

I generally suspect cck will handle setting the default so you don't really need to bother with the mess, perhaps, but didn't spend too much time looking into it. :>