platform/x86: asus-wmi: Fix keyboard brightness cannot be set to 0

Some of ASUS laptops like UX431FL keyboard backlight cannot be set to
brightness 0. According to ASUS' information, the brightness should be
0x80 ~ 0x83. This patch fixes it by following the logic.

Fixes: e9809c0b

 ("asus-wmi: add keyboard backlight support")
Signed-off-by: Jian-Hong Pan <jian-hong@endlessm.com>
Reviewed-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>