eeepc-laptop: report brightness control events via the input layer

This maps the brightness control events to one of two keys, either
KEY_BRIGHTNESSDOWN or KEY_BRIGHTNESSUP, as needed.

Some mapping has to be done due to the fact that the BIOS reports them as
<base value> + <current brightness index>; the selection is done according to
the sign of the change in brightness (if this is 0, no keypress is reported).

(Ref. http://lists.alioth.debian.org/pipermail/debian-eeepc-devel/2009-April/002001.html)

Signed-off-by: Darren Salt <linux@youmustbejoking.demon.co.uk>
Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Len Brown <len.brown@intel.com>