asus-wmi: add scalar board brightness adj. support

Some ASUS ET2012E/I All-in-One machines that use a scalar board
to control the brightness, and they only accept brightness up and down
command. So, I introduced a get_scalar_command() function to pass the
command to the scalar board through WMI.

Besides, we have to store the brightness value locally, for we need the
old value to know the brightness value is increasing or decreasing.

BTW, since there is no way to retrieve the actual brightness(it would be
a fixed value), and the max brightness value would be fixed to 1, so we
have to keep passing the brightness up/down command when we reached the
max brightness value or 0.

Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Corentin Chary <corentin.chary@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>