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_e1.c b/src/libosmo-mgcp/mgcp_e1.c
index 7e810bc..effbe77 100644
--- a/src/libosmo-mgcp/mgcp_e1.c
+++ b/src/libosmo-mgcp/mgcp_e1.c
@@ -197,7 +197,7 @@
 	const uint8_t *ptr_ft;
 	enum osmo_trau_frame_type ft;
 
-	rate_ctr_inc(&rate_ctrs->ctr[E1_I460_TRAU_MUX_EMPTY_CTR]);
+	rate_ctr_inc(rate_ctr_group_get_ctr(rate_ctrs, E1_I460_TRAU_MUX_EMPTY_CTR));
 
 	/* Choose an appropiate idle frame type */
 	ft = endp->e1.trau_rtp_st->type;
@@ -304,7 +304,7 @@
 	msgb_free(msg);
 	return;
 skip:
-	rate_ctr_inc(&rate_ctrs->ctr[E1_I460_TRAU_RX_FAIL_CTR]);
+	rate_ctr_inc(rate_ctr_group_get_ctr(rate_ctrs, E1_I460_TRAU_RX_FAIL_CTR));
 	msgb_free(msg);
 	return;
 }
@@ -679,7 +679,7 @@
 
 	return 0;
 skip:
-	rate_ctr_inc(&rate_ctrs->ctr[E1_I460_TRAU_TX_FAIL_CTR]);
+	rate_ctr_inc(rate_ctr_group_get_ctr(rate_ctrs, E1_I460_TRAU_TX_FAIL_CTR));
 	msgb_free(msg_tf);
 	return -1;
 }