ynl: add missing pieces to ethtool spec to better match uapi header

- __ETHTOOL_UDP_TUNNEL_TYPE_CNT and render max
- skip rendering stringset (empty enum)
- skip rendering c33-pse-ext-state (defined in ethtool.h)
- rename header flags to ethtool-flag-
- add attr-cnt-name to each attribute to use XXX_CNT instead of XXX_MAX
- add unspec 0 entry to each attribute
- carry some doc entries from the existing header
- tcp-header-split

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241204155549.641348-5-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>