[media] v4l2-mem2mem.h: make kernel-doc parse v4l2-mem2mem.h again

The kernel-doc C parser doesn't like opaque structures. So,
document it on another way.

This should get rid of this warning:
	./include/media/v4l2-mem2mem.h:62: error: Cannot parse struct or union!

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