[media] smiapp: Scaling goodness is signed

The "best" value was unsigned however, leading to signed-to-unsigned
comparison and wrong results. Possibly only on a newer GCC. Fix this by
making the best value signed as well.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>