tools: ynl: make definitions optional again

definitions are optional, commit in question breaks cli for ethtool.

Fixes: 6517a60b03 ("tools: ynl: move the enum classes to shared code")
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/nlspec.py

@@ -387,7 +387,8 @@ class SpecFamily(SpecElement):
     def resolve(self):
         self.resolve_up(super())
 
-        for elem in self.yaml['definitions']:
+        definitions = self.yaml.get('definitions', [])
+        for elem in definitions:
             if elem['type'] == 'enum' or elem['type'] == 'flags':
                 self.consts[elem['name']] = self.new_enum(elem)
             else: