Changes

trau_frame: D145_SYNC does not follow EDATA format

14.5 kbit/s data service uses a special protocol: the CCU and the
TRAU exchange D145_SYNC frames first, conveying only alignment of
320-bit TRAU frames, then switch to EDATA frames that can carry
payload.  (EDATA frames don't have enough sync pattern bits to be
reliable against false alignment.)

Per TS 48.060 section 5.5.3, D145_SYNC frames are structured like
"regular" (lower rates) data frames with all payload bits set to 1,
_not_ like EDATA frames.  Therefore:

* osmo_trau_frame_decode_16k() should decode D145_SYNC frames like
  regular data frames, not like EDATA;

* osmo_trau_frame_encode() given OSMO_TRAU16_FT_D145_SYNC should
  produce a frame with correct sync pattern and all-1 payload bits,
  but with control bits up to C15 per TS 48.060 section 5.5.3.

Change-Id: I7c743996fc620227438225ffda419217881034d1