fix typo in platform API base class (sonic-net#25)

Author: keboliu

sonic_platform_base/chassis_base.py

@@ -220,4 +220,4 @@ def get_watchdog(self):
             An object derived from WatchdogBase representing the hardware
             watchdog device
         """
-        return _watchdog
+        return self._watchdog

sonic_platform_base/psu_base.py

@@ -29,7 +29,7 @@ def get_fan(self):
             An object dervied from FanBase representing the fan module
             contained in this PSU
         """
-        return _fan
+        return self._fan
 
     def set_status_led(self, color):
         """