tools: ynl: fix converting flags to names after recent cleanup

I recently cleaned up specs to not specify enum-as-flags
when target enum is already defined as flags.
YNL Python library did not convert flags, unfortunately,
so this caused breakage for Stan and Willem.

Note that the nlspec.py abstraction already hides the differences
between flags and enums (value vs user_value), so the changes
are pretty trivial.

Fixes: 0629f22e ("ynl: netdev: drop unnecessary enum-as-flags")
Reported-and-tested-by: Willem de Bruijn <willemb@google.com>
Reported-and-tested-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/all/ZS10NtQgd_BJZ3RU@google.com/
Link: https://lore.kernel.org/r/20231016213937.1820386-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>