[media] v4l2-flash-led-class.h: document v4l2_flash_ops

Fix this warning:
	./include/media/v4l2-flash-led-class.h:103: WARNING: c:type reference target not found: v4l2_flash_ops

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>