updatefrequency should not be saved into duration

When updatefrequency is set and is a string, its value should be saved
into updateFrequency, and it shouldn't override duration.

Signed-off-by: Oleg Bulatov <oleg@bulatov.me>

registry/storage/driver/middleware/cloudfront/middleware.go

@@ -127,7 +127,7 @@ func newCloudFrontStorageMiddleware(storageDriver storagedriver.StorageDriver, o
 			if err != nil {
 				return nil, fmt.Errorf("invalid updatefrequency: %s", err)
 			}
-			duration = updateFreq
+			updateFrequency = updateFreq
 		}
 	}