In port_set_mode(), verify the uncached _port_get_mode()

Otherwise we'll get old data and things will fail.

Change-Id: Ibeebf197799ef98ac655e5fc0f14c438115e1f0d
diff --git a/drivers/CiscoSX300.py b/drivers/CiscoSX300.py
index 54d318e..188926b 100644
--- a/drivers/CiscoSX300.py
+++ b/drivers/CiscoSX300.py
@@ -212,7 +212,7 @@
             self._end_configure()
 
             # Validate it happened
-            read_mode = self.port_get_mode(port)
+            read_mode = self._port_get_mode(port)
             if read_mode != mode:
                 raise IOError("Failed to set mode for port %s" % port)