mtd: mtdoops: replace simple_strtoul with kstrtouint

mtd: mtdoops: replace simple_strtoul with kstrtouint

Replace deprecated simple_strtoul with kstrtouint for better error
handling and type safety. The kstrtouint function provides stricter
validation, automatically rejecting inputs like "123abc" that
simple_strtoul would partially accept.

Using kstrtouint avoids unsigned long to int conversion and is more
appropriate for MTD device indices which are non-negative integers.

Signed-off-by: Haoyu Lu <hechushiguitu666@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>