It's easier to refactor these parsers if the return value gets assigned
only once, just like every other MTD partition parser.
This prepares for making the second arg to the parse_fn() const. This is
OK if we construct the partitions completely first, and assign them to
the return pointer only after we're done modifying them.
Signed-off-by: Brian Norris <computersforpeace@gmail.com> Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>