Fixes smatch warning:

drivers/media/v4l2-core/v4l2-subdev.c:978 __v4l2_subdev_state_alloc() warn: Please consider using kvcalloc instead
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-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>