Changes

trau_rtp_conv: use new RA2 functions for CSD

osmo_trau2rtp() and osmo_rtp2trau() include CSD support; the RTP
format in CSD modes is RFC 4040 clearmode like the interface that
exists on the 64 kbit/s side of a traditional TRAU.  RA2 packing
and unpacking constitute required steps in these conversions -
use newly factored-out functions.

Change-Id: Ia157989579ced866b1e7abfc789d69ae489224e9