tools: ynl: fix enum-as-flags in the generic CLI

Lorenzo points out that the generic CLI is broken for the netdev
family. When I added the support for documentation of enums
(and sparse enums) the client script was not updated.
It expects the values in enum to be a list of names,
now it can also be a dict (YAML object).

Reported-by: Lorenzo Bianconi <lorenzo@kernel.org>
Fixes: e4b48ed4

 ("tools: ynl: add a completely generic client")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>