sony-laptop: Prepare the platform driver for multiple users.

Both the SNC and SPIC device drivers will create attributes and thus
there's the need to have an internal usage count to avoid
re-registering or de-registering at the wrong time.

Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Len Brown <len.brown@intel.com>