Now that kernel-doc can hanle nested structs/unions, describe
such fields at w1_netlink_message_types.

Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>