sh: intc: Replace simple_strtoul() with kstrtoul()

The function simple_strtoul() performs no error checking
in scenarios where the input value overflows the intended
output variable.

We can replace the use of simple_strtoul() with the safer
alternative kstrtoul(). This also allows us to print an
error message in case of failure.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>