Update filter_ops.py (#1657)

currently for every mode value other than None - a 'NameError: name 'upper' is not defined' was thrown due to misuse of the 'upper' method

Author: trabenx

tensorflow_io/python/experimental/filter_ops.py

@@ -22,7 +22,7 @@
 def pad(input, ksize, mode, constant_values):
     input = tf.convert_to_tensor(input)
     ksize = tf.convert_to_tensor(ksize)
-    mode = "CONSTANT" if mode is None else upper(mode)
+    mode = "CONSTANT" if mode is None else str.upper(mode)
     constant_values = (
         tf.zeros([], dtype=input.dtype)
         if constant_values is None