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/src/libosmo-mgcp/mgcp_network.c b/src/libosmo-mgcp/mgcp_network.c
index e03c100..79ddf8a 100644
--- a/src/libosmo-mgcp/mgcp_network.c
+++ b/src/libosmo-mgcp/mgcp_network.c
@@ -66,8 +66,8 @@
 	struct rate_ctr_group *mgw_stats = endp->trunk->ratectr.all_rtp_conn_stats;
 
 	/* add to both the per-connection and the global stats */
-	rate_ctr_add(&conn_stats->ctr[id], inc);
-	rate_ctr_add(&mgw_stats->ctr[id], inc);
+	rate_ctr_add(rate_ctr_group_get_ctr(conn_stats, id), inc);
+	rate_ctr_add(rate_ctr_group_get_ctr(mgw_stats, id), inc);
 }
 
 static void rtpconn_rate_ctr_inc(struct mgcp_conn_rtp *conn_rtp, struct mgcp_endpoint *endp, int id)
@@ -1475,8 +1475,8 @@
 					sizeof(struct sockaddr_in)));
 
 	/* Increment RX statistics */
-	rate_ctr_inc(&conn_src->rate_ctr_group->ctr[RTP_PACKETS_RX_CTR]);
-	rate_ctr_add(&conn_src->rate_ctr_group->ctr[RTP_OCTETS_RX_CTR], msgb_length(msg));
+	rate_ctr_inc(rate_ctr_group_get_ctr(conn_src->rate_ctr_group, RTP_PACKETS_RX_CTR));
+	rate_ctr_add(rate_ctr_group_get_ctr(conn_src->rate_ctr_group, RTP_OCTETS_RX_CTR), msgb_length(msg));
 	/* FIXME: count RTP and RTCP separately, also count IuUP payload-less separately */
 
 	/* Forward a copy of the RTP data to a debug ip/port */