mtd: parser: imagetag: fix error codes in bcm963xx_parse_imagetag_partitions()

If the kstrtouint() calls fail, then this should return a negative
error code but it currently returns success.

Fixes: dd84cb02 ("mtd: bcm63xxpart: move imagetag parsing to its own parser")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/YBKFtNaFHGYBj+u4@mwanda