tools: ynl: generate code for the ethtool family

Generate the protocol code for ethtool. Skip the stats
for now, they are the only outlier in terms of complexity.
Stats are a sort-of semi-polymorphic (attr space of a nest
depends on value of another attr) or a type-value-scalar,
depending on how one wants to look at it...
A challenge for another time.

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