[media] media: entity: Be vocal about failing sanity checks

Commit 3801bc7d ("[media] media: Media Controller fix to not let
stream_count go negative") added a sanity check for negative stream_count,
but a failure of the check remained silent. Make sure the failure is
noticed.

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