tools: ynl-gen: don't init enum checks for classic netlink

tools: ynl-gen: don't init enum checks for classic netlink

rt-link has a vlan-protocols enum with:

   name: 8021q     value: 33024
   name: 8021ad    value: 34984

It's nice to have, since it converts the values to strings in Python.
For C, however, the codegen is trying to use enums to generate strict
policy checks. Parsing such sparse enums is not possible via policies.

Since for classic netlink we don't support kernel codegen and policy
generation - skip the auto-generation of checks from enums.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250429154704.2613851-12-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>