sony-laptop: add smart connect control function

The current value is not available through the SNC device and therefore
the attribute is writable only.

Signed-off-by: Javier Achirica <jachirica@gmail.com>
Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>