Use new stat item/ctr getter APIs

Generated with spatch with this and similat snippets:
"""
@@
expression E1, E2;
@@
- &E2->ctr[E1]
+ rate_c

Change-Id: I53b75ea8a88bc1ae4ceb479ed272865054de9665
diff --git a/tests/mgcp/mgcp_test.c b/tests/mgcp/mgcp_test.c
index 620b0d1..74ecfe8 100644
--- a/tests/mgcp/mgcp_test.c
+++ b/tests/mgcp/mgcp_test.c
@@ -1071,7 +1071,7 @@
 				    "test-connection");
 		conn = mgcp_conn_get_rtp(&endp, _conn->id);
 		state = &conn->state;
-		packets_rx = &conn->rate_ctr_group->ctr[RTP_PACKETS_RX_CTR];
+		packets_rx = rate_ctr_group_get_ctr(conn->rate_ctr_group, RTP_PACKETS_RX_CTR);
 
 		state->stats.initialized = 1;
 		state->stats.base_seq = pl_test_dat[i].base_seq;