mtd: ofpart: assign return argument exactly once

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>