media: mc: Set bus_info in media_device_init()

Set bus_info field based on struct device in media_device_init() and
remove corresponding code from drivers.

Also update media_device_init() documentation: the dev field must be now
initialised before calling it.

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>