Sony-laptop: Fix sparse warning (make undeclared var static)

Fix the following sparse warning:

sony-laptop.c:1035:29: warning: symbol 'sony_bl_props' was not declared. Should it be static?

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>