platform/x86: asus-wmi: Refactor charge threshold to use the battery hooking API

At the same time use the official naming for the knobs.

Tested on a Zenbook UX430UNR.

Signed-off-by: Kristian Klausen <kristian@klausen.dk>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>