util/agents/usb_hid_relay: Fix agent errors

* get_output() should return some result;
* should attach kernel driver back in the destructor otherwise HIDRAW
  device will not appear if it has existed.

Signed-off-by: Roman Kuznecov <zubastikiko@gmail.com>

Author: cidlik

labgrid/util/agents/usb_hid_relay.py

@@ -100,7 +100,10 @@ def set_output(self, number, status):
 
     def get_output(self, number):
         with self._claimed():
-            self._get_output(number)
+            return self._get_output(number)
+
+    def __del__(self):
+        self._dev.attach_kernel_driver(0)
 
 
 _relays = {}