Fix ep size mistake for dfn-6
Fixes the dfn footprint with regards to the problem described in https://github.com/KiCad/kicad-footprints/issues/678. The original footprint has been added in https://github.com/KiCad/kicad-footprints/pull/539. The symbol side of things is done in https://github.com/KiCad/kicad-symbols/pull/679.