kernel: mtdsplit_minor: return 0 if not fatal

kernel: mtdsplit_minor: return 0 if not fatal

Introduced with Linux 6.7, in commit:
5c2f7727d437 ("mtd: mtdpart: check for subpartitions parsing result"),
when a parser returns an error, this will be passed up, and
consequently, all parent mtd partitions get torn down.

Adjust the MiNOR mtdsplit driver to only return an error if there is a
critical problem in reading from the mtd device or allocating memory.
Otherwise return 0 to indicate that no partitions were found.
Also add logging to indicate what went wrong.

This mtdsplit parser makes a very limited check of the first YAFFS
header. For example, this will not match expectations when initially booting
an initramfs image with OEM on MTD.

Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>
Acked-by: Thibaut VARENE <hacks@slashdirt.org>
Link: https://github.com/openwrt/openwrt/pull/16780
Signed-off-by: Robert Marko <robimarko@gmail.com>