mfd: ab8500-debug: Convert to kstrtoul_from_user

Use kstrtoul_from_user for getting an unsigned long from userspace
which is less error prone.

Signed-off-by: srinidhi kasagar <srinidhi.kasagar@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Jonas ABERG <jonas.aberg@stericsson.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>