platform/x86: asus-wmi: Fix preserving keyboard backlight intensity on load

The error code and return value are mixed up. The intensity is always set
to 0 on load as kbd_led_read returns either 0 or negative value. To
reproduce set backlight to maximum, reload driver and try to increase it
using keyboard hotkey, the intensity will drop as a result. Correct the
implementation.

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