media: video-mux: Add missing media_entity_cleanup upon async register fail

Although media_entity_pads_init has been called, a call to
media_entity_cleanup in the cleanup code was missing.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>