mtd: partitions: rename MTD parser get/put

We're going to reuse put_partition_parser(), so let's fix up the prefix
naming a bit, to hopefully be more consistent. Also make convert to a
true C function instead of a macro.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>