platform/x86: asus-wmi: Do not disable keyboard backlight on unloading

The keyboard backlight is automatically disabled when the module is
unloaded as it is exposed as a ledclass device. Change this behavior to
ignore setting brightness when the device is in unloading state.

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