Fix bug that missing the self argument in get_power_state_controller()

A trivial bug that missing the self argument.

BUG=chrome-os-partner:18541
TEST=None, trivial fix

Change-Id: Ic051af234b50180c95e9ecbe2326ccc57c9887b8
Reviewed-on: https://gerrit.chromium.org/gerrit/47228
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Queue: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
diff --git a/server/cros/servo/servo.py b/server/cros/servo/servo.py
index 582c919..3d6126a 100644
--- a/server/cros/servo/servo.py
+++ b/server/cros/servo/servo.py
@@ -146,7 +146,7 @@
             self._scp_cmd_template = 'scp -r %s ' % common_options
             self._scp_cmd_template += '%s ' + 'root@' + servo_host + ':%s'
 
-    def get_power_state_controller():
+    def get_power_state_controller(self):
         """Return the power state controller for this Servo.
 
         The power state controller provides board-independent