platform/x86: asus-wmi: Refactor WMI event handling

Refactor WMI event handling into separate functions for getting the event
code and handling the retrieved event code as a preparation for
introduction of WMI event queue support.

Signed-off-by: Yurii Pavlovskyi <yurii.pavlovskyi@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>