turn-on-haskell-simple-indent breaks auto-fill-mode for comments

When writing comments with auto-fill-mode and turn-on-haskell-simple-indent, auto-fill makes the comments staircase, like this:

```
-- comment line 1, lorem ipsum etc.
  -- comment line 2
    -- comment line 3
```

When no haskell mode indentation is on, auto-fill-mode does what it should for comments.
