media: mc: media_device_init() initialises a media_device, not media_entity

The documentation for media_device_init() had several references to
(struct) media_entity where it should have referred to struct media_device
instead. Fix this.

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@kernel.org>