Clean up minor style descrepency in plot.py

plot.py

@@ -27,9 +27,9 @@ class _PadExclusion:
             if ((self.__name is None or
                     self.__name == self.__normalize_name(pad.GetName())) and
                     pad.IsOnLayer(layer_num)):
-                layers = LSET()
                 pad_mask = int(pad.GetLayerSet().FmtHex().replace('_', ''), 16)
                 pad_hex = hex(pad_mask & layer_mask)
+                layers = LSET()
                 layers.ParseHex(pad_hex, len(pad_hex))
                 pad.SetLayerSet(layers)
                 count += 1