Add logging for the port mode cache code

Change-Id: I993c919d7af0fc801547fb3b9eb696983d712f9f
diff --git a/drivers/common.py b/drivers/common.py
index 4c4b189..f0d2128 100644
--- a/drivers/common.py
+++ b/drivers/common.py
@@ -130,10 +130,12 @@
         if not self._is_port_name_valid(port):
             raise InputError("Port name %s not recognised" % port)
         if port in self._port_modes:
+            logging.debug("port_get_mode: returning mode %s from cache for port %s", self._port_modes[port], port)
             return self._port_modes[port]
         else:
             mode = self._port_get_mode(port)
             self._port_modes[port] = mode
+            logging.debug("port_get_mode: found mode %s for port %s, adding to cache", self._port_modes[port], port)
             return mode
 
     # Wrappers to adapt logging for pexpect when we've configured on a