osmux: Fix incorrect rate_ctr_group used in mgcp_osmux.c

During development of the counters, they were first added to the same
RTP counter group and later on split, but some places still used the
RTP related functions to increment the counters.
Let's add a seaprate set of helpers to update the correct osmux counter
group.

Fixes: 582c2bf7b0b3f6aabfb048a2d71c5fd9c60a105e

Change-Id: Ia2e5601c7d476b79afd95032dbc019517f8209af
diff --git a/src/libosmo-mgcp/mgcp_ratectr.c b/src/libosmo-mgcp/mgcp_ratectr.c
index bf3d9d3..abf3883 100644
--- a/src/libosmo-mgcp/mgcp_ratectr.c
+++ b/src/libosmo-mgcp/mgcp_ratectr.c
@@ -151,7 +151,7 @@
 	.group_description = "aggregated statistics for all osmux connections",
 	.class_id = 1,
 	.num_ctr = ARRAY_SIZE(all_osmux_conn_rate_ctr_desc),
-	.ctr_desc = all_rtp_conn_rate_ctr_desc
+	.ctr_desc = all_osmux_conn_rate_ctr_desc
 };
 
 /*! allocate global rate counters