tools: ynl-gen: respect attr-cnt-name at the attr set level

Davide reports that we look for the attr-cnt-name in the wrong
object. We try to read it from the family, but the schema only
allows for it to exist at attr-set level.

Reported-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/all/CAKa-r6vCj+gPEUKpv7AsXqM77N6pB0evuh7myHq=585RA3oD5g@mail.gmail.com/
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20231025182739.184706-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>