ynl-gen-c.py: filter rendering of validate field values for split ops

For split ops, do and dump has different meaningful values in
validate field.

Fix the rendering to allow the values per op type as follows:
do: strict
dump: dump, strict-dump

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230803111340.1074067-3-jiri@resnulli.us


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