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
2 files changed