mfd: Use kstrtoul_from_user in ab8500

This patch replaces the code for getting an unsigned long from a
userspace buffer by a simple call to kstroul_from_user.
This makes it easier to read and less error prone.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>