platform/x86: hp-wmi: Convert simple_strtoul() to kstrtou32()

First of all, unsigned long can overflow u32 value on 64-bit machine.
Second, simple_strtoul() doesn't check for overflow in the input.

Convert simple_strtoul() to kstrtou32() to eliminate above issues.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>