stats: use libosmocore rate counter for in/out_stream.err_ts_counter

The two counters: in_stream.err_ts_counter and out_stream.err_ts_counter
are still handcoded. To make them better accessible they should
 be replaced with libosmocore rate counters.

- replace state.in_stream.err_ts_counter with libosmocore rate counter
- replace state.out_stream.err_ts_counter with libosmocore rate counter

Change-Id: I9fbd65bf2f4d1e015a05996db4c1f7ff20be2c95
Related: OS#2517
diff --git a/src/libosmo-mgcp/mgcp_stat.c b/src/libosmo-mgcp/mgcp_stat.c
index 581130c..cc723bb 100644
--- a/src/libosmo-mgcp/mgcp_stat.c
+++ b/src/libosmo-mgcp/mgcp_stat.c
@@ -87,9 +87,9 @@
 	if (conn->conn->endp->cfg->osmux != OSMUX_USAGE_OFF) {
 		/* Error Counter */
 		nchars = snprintf(str, str_len,
-				  "\r\nX-Osmo-CP: EC TI=%u, TO=%u",
-				  conn->state.in_stream.err_ts_counter,
-				  conn->state.out_stream.err_ts_counter);
+				  "\r\nX-Osmo-CP: EC TI=%lu, TO=%lu",
+				  conn->state.in_stream.err_ts_ctr->current,
+				  conn->state.out_stream.err_ts_ctr->current);
 		if (nchars < 0 || nchars >= str_len)
 			goto truncate;