sony-laptop: new keyboard backlight handle

Add support for handle 0x0143 (Vaio SA/SB/SC, CA/CB) and rework the code
to be hable to support different handles for the keyboard backlight
function.

[malattia@linux.it: group function specific variables in a struct]

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