Add a missing yield in device key upload

(this would only very very rarely actually be a useful thing, so the main
problem was the logcontext leak...)

synapse/handlers/e2e_keys.py

@@ -316,7 +316,7 @@ class E2eKeysHandler(object):
         # old access_token without an associated device_id. Either way, we
         # need to double-check the device is registered to avoid ending up with
         # keys without a corresponding device.
-        self.device_handler.check_device_registered(user_id, device_id)
+        yield self.device_handler.check_device_registered(user_id, device_id)
 
         result = yield self.store.count_e2e_one_time_keys(user_id, device_id)