Changes

rtp2trau FR & EFR: disallow BFI in DL

osmo_rtp2trau() accepts TW-TS-001 extended RTP payload format for
FR and EFR, converting to either TRAU-DL or TRAU-UL.  This extended
format is required for TRAU-UL/TFO frame output, but it is also
accepted for conversion to TRAU-DL in order to support the current
form of OsmoMGW-E1 that has no TFO transform between RTP input and
TDM-timed calls to osmo_rtp2trau().

TW-TS-001 can represent not only good frames, but also bad ones -
however, bad frames on Abis are valid only in UL, not in DL.  Prior
to the present patch, if a BFI-marked frame were fed to osmo_rtp2trau()
for conversion to TRAU-DL, the Bad Frame Indicator was dropped and
the garbage payload bits were emitted as if they were a good frame -
bad choice.  Reject BFI frames in TRAU-DL mode, return -EINVAL.

Change-Id: Ic63943d4bde9902b27e4d8fe9a5fb6ccecbf36c6