Fix attrib plugin deregistration

As the comparison method used for find what to de-register was
wrong, it was causing the btd_device reference that the attrib
plugin was keeping never to be dropped.
diff --git a/attrib/client.c b/attrib/client.c
index 8e96af4..69e4fb8 100644
--- a/attrib/client.c
+++ b/attrib/client.c
@@ -167,7 +167,7 @@
 	const struct gatt_service *gatt = a;
 	const struct btd_device *dev = b;
 
-	return gatt->dev == dev;
+	return gatt->dev != dev;
 }
 
 static int characteristic_handle_cmp(gconstpointer a, gconstpointer b)