Changes

E1: replace idle_tf_efr[] with a better version

The only truly correct way to pass FR/HR/EFR traffic from an
incoming RTP stream to TRAU-DL frames is to apply the transform
of TS 28.062 section C.3.2.1.1, originally specified for TFO
but also necessary in the non-3GPP-specified case of GSM TrFO
as it happens here.  However, no FOSS or even published-source
implementation of TFO transform for EFR exists at the present time
(no implementations at all are known outside of TRAU DSP firmware),
hence the general architectural approach of OsmoMGW-E1, flawed
as it is, has to remain for now.  This flawed architecture consists
of passing through unaltered all valid codec frames that are
received in RTP, and inserting fixed dummy TRAU-DL frames when no
RTP-derived ones are available.

The original dummy TRAU-DL frame for EFR exhibited the following
defects:

* D1 bit was set to 0, even though TS 48.060 section 5.5.1.1.2
  states it shall be 1;

* All 5 EFR frame CRCs were invalid: each 3-bit CRC was 000, even
  though the correct TS 48.060 section 5.5.1.1.2 CRC from an
  all-zeros payload would be 111;

* Even if the 5 CRCs were fixed, transmitting an EFR frame of all
  zero bits to the MS won't produce good results - it is a bad
  choice of dummy filler;

* The setting of C-bits (defined in TS 48.060 section 5.5.1.1.1)
  was a mish-mash between TRAU-UL and TRAU-DL frame formats,
  thereby not matching the standard TRAU-DL C-bits fill produced
  by osmo_rtp2trau().

The replacement fill frame introduced here differs as follows:

* The payload bit content is the decoder homing frame (DHF) of
  TS 46.060 section 8.2 Table 7 - the best we can do in the absence
  of a proper TFO transform for EFR;

* The full TRAU-DL frame (C-bits, CRC etc) was generated by passing
  said EFR DHF through osmo_rtp2trau(), using this ad hoc tool:

https://gitea.osmocom.org/themwi/dummy-dl-frames

Change-Id: I5a33a1c9ddf1372f91870d61b5eafac4729ee458