netlink: specs: ethtool: define header-flags as an enum

Recent changes added header flags to the spec.
Use an enum instead of defines for more seamless codegen.

[Jakub: drop the already applied parts and rewrite message]

Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Link: https://lore.kernel.org/r/20240403212931.128541-6-rrameshbabu@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>