media: rkisp1: stats: Simplify (un)registration

Simplify error handling at registration time as media_entity_cleanup()
can be called on an uninitialized entity, and make
rkisp1_stats_unregister() safe to be called on an unregistered stats
node to prepare for simplification of error handling at probe time.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>