[media] media: entity: Fix stream count check

There's a sanity check for the stream count remaining positive or zero on
error path, but instead of performing the check on the traversed entity it
is performed on the entity where traversal ends. Fix this.

Fixes: commit 3801bc7d ("[media] media: Media Controller fix to not let stream_count go negative")

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>