platform/x86: asus-wmi: Simplify the keyboard brightness updating process

The original asus-wmi queues a work which calls the ACPI/WMI methods to
update the keyboard LED brightness.  Similar drivers - acer-wmi,
dell-wmi-led just call the ACPI/WMI methods directly without workqueues.

This patch simplifies the keyboard brightness updating process which
calls the kbd_led_update function directly without workqueue in
asus-wmi.

Signed-off-by: Jian-Hong Pan <jian-hong@endlessm.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>