netlink: specs: ethtool: untangle UDP tunnels and cable test a bit

UDP tunnel and cable test messages have a lot of nests,
which do not match the names of the enum entries in C uAPI.
Some of the structure / nesting also looks wrong.

Untangle this a little bit based on the names, comments and
educated guesses, I haven't actually tested the results.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>