media: v4l2-subdev: Fix missing kerneldoc for client_caps

Add missing kernel doc for the new 'client_caps' field in struct
v4l2_subdev_fh.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Fixes: f57fa295 ("media: v4l2-subdev: Add new ioctl for client capabilities")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>