tools: ynl-gen: use enum user type for members and args

Commit 30c90200

 ("tools: ynl-gen: use enum name from the spec")
added pre-cooked user type for enums. Use it to fix ignoring
enum-name provided in the spec.

This changes a type in struct ethtool_tunnel_udp_entry but is
generally inconsequential for current families.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231213231432.2944749-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>