ACPI: x86: Refactor _UID handling to use acpi_dev_uid_to_integer()

ACPI utils provide acpi_dev_uid_to_integer() helper to extract _UID as
an integer. Use it instead of custom approach.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>