media: video-mux: fix error paths

Move notifier cleanup into video_mux_async_register() to avoid calling
v4l2_async_nf_unregister() when v4l2_async_subdev_nf_register() failed.
In case video_mux_async_register() fails, call media_entity_cleanup().

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>