[PATCH] sony-laptop: limit brightness range to DSDT provided ones

The new style brightness control provides an operating range of 9 values
(seems consistent over a large number of models sharing the same
brightness control methods).
Read and use the minimum and maximum values to limit the backlight
interface between those boundaries.

Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Matthew Garrett <mjg@redhat.com>