openflow_input: fix naming of group type in group desc stats entry

The C backend blacklists the name "type" so accessors aren't generated.
"group_type" works and is consistent with the other classes.
diff --git a/test_data/of13/group_desc_stats_reply.data b/test_data/of13/group_desc_stats_reply.data
index 34c0d50..5e95a67 100644
--- a/test_data/of13/group_desc_stats_reply.data
+++ b/test_data/of13/group_desc_stats_reply.data
@@ -49,7 +49,7 @@
     flags=0,
     entries=[
         ofp.group_desc_stats_entry(
-            type=ofp.OFPGT_FF,
+            group_type=ofp.OFPGT_FF,
             group_id=1,
             buckets=[
                 ofp.bucket(
@@ -66,4 +66,4 @@
                     actions=[
                         ofp.action.output(port=5, max_len=0),
                         ofp.action.output(port=6, max_len=0)])]),
-        ofp.group_desc_stats_entry(type=ofp.OFPGT_FF, group_id=2, buckets=[])])
+        ofp.group_desc_stats_entry(group_type=ofp.OFPGT_FF, group_id=2, buckets=[])])