mtd: mtdpart: check for subpartitions parsing result

parse_mtd_partitions() may return an error so it should be checked and
optionally passed up

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230927202657.27169-1-zajec5@gmail.com