utils/asyn: Fix memoized_method.__set_name__

Set the "_name" attribute rather than trying to set the "name" read-only
property.

devlib/utils/asyn.py

@@ -289,7 +289,7 @@ class memoized_method:
         raise RuntimeError("Cannot monkey-patch a memoized function")
 
     def __set_name__(self, owner, name):
-        self.name = name
+        self._name = name
 
 
 def asyncf(f):