There are two similar functions handling deletion. One handles single
partition and another the whole MTD flash device. They share (duplicate)
some code so it makes sense to add a small helper for that part.

Function del_mtd_partitions has been moved a bit to keep all deleting
stuff together.
Signed-off-by: NRafał Miłecki <rafal@milecki.pl>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>