Changes

rtp2trau HR: add support for TRAU-8k-UL frame output

When osmo_rtp2trau() function is used to pass traffic to an E1 BTS,
it only needs to generate TRAU-DL frames, not TRAU-UL.  OTOH,
ability to generate TRAU-UL frames is needed for TFO, i.e., for
software implementation of TFO-capable speech transcoders.
osmo_rtp2trau() already supports TRAU-UL frame output for FR and
EFR codecs, but not for HR codec in TRAU-8k framing - add the
missing support.  (Out of the two possible TRAU frame formats for
GSM-HR codec, TFO always uses TRAU-8k format.)

Change-Id: I288cfa7d467125f7732ef1d0ef83b933e41da536