tools: ynl-gen: don't override pure nested struct

For pure structs (parsed nested attributes) we track what
forms of the struct exist in request and reply directions.
Make sure we don't overwrite the recorded struct each time,
otherwise the information is lost.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/ynl-gen-c.py

@@ -825,7 +825,8 @@ class Family(SpecFamily):
                     inherit = set()
                     nested = spec['nested-attributes']
                     if nested not in self.root_sets:
-                        self.pure_nested_structs[nested] = Struct(self, nested, inherited=inherit)
+                        if nested not in self.pure_nested_structs:
+                            self.pure_nested_structs[nested] = Struct(self, nested, inherited=inherit)
                     if attr in rs_members['request']:
                         self.pure_nested_structs[nested].request = True
                     if attr in rs_members['reply']: