ACPI / osl: Propagate actual error code for kstrtoul()

There is no need to override the error code returned by kstrtoul().
Propagate it directly to the caller.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>