coding: properly handle AFS_SID_UPDATE frames in DTX mode

There are two similar values in enum gsm0503_amr_dtx_frames:

  * AFS_SID_UPDATE - precursor of SID UPDATE,
  * AFS_SID_UPDATE_CN - the actual SID UPDATE.

The former is internally used by libosmocoding to mark the current
frame as a precursor of the actual SID UPDATE frame - the later.

  +---+---+---+---+---+---+---+---+
  | _ | _ | _ | _ | a | b | c | d |  AFS_SID_UPDATE
  +---+---+---+---+---+---+---+---+
  | a | b | c | d | _ | _ | _ | _ |  AFS_SID_UPDATE_CN
  +---+---+---+---+---+---+---+---+

This is required because function gsm0503_tch_afs_decode_dtx() is
invoked by TDMA scheduler on every 4th received burst, while the
burst buffer is 8 bursts wide.

Currently, whenever gsm0503_detect_afs_dtx_frame() detects an
AFS_SID_UPDATE frame, we still attempt to decode it as a speech
or data below in gsm0503_tch_afs_decode_dtx().  This is indeed
a bug, which results in unexpected BER values:

  * expected BER 0/212,
  * actual BER 252/448.

We should return immediately once we have detected an AFS_SID_UPDATE.
This patch fixes unexpected BER-SUB values during DTXu silence periods.

Change-Id: I813081a4c0865958eee2496fe251ae17235ac842
Related: SYS#5853

src/coding/gsm0503_coding.c

@@ -2222,6 +2222,7 @@ int gsm0503_tch_afs_decode_dtx(uint8_t *tch_data, const sbit_t *bursts,
 			tch_amr_reassemble(tch_data, conv, 39);
 			len = 5;
 			goto out;
+		case AFS_SID_UPDATE:
 		case AFS_ONSET:
 			len = 0;
 			goto out;