mtd: mtdsuper: replace simple_strtoul with kstrtouint

mtd: mtdsuper: replace simple_strtoul with kstrtouint

Modernize string-to-number conversion in mtdsuper.c by replacing
simple_strtoul with kstrtouint. This change provides proper type safety
for MTD device numbers which are non-negative integers.

Using kstrtouint avoids unsigned long to int conversion and is more
appropriate for device indices. The debug output format specifier is
updated to %u for unsigned int.

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