AMR DTX: lchan->tch.last_sid is never populated with save_last_sid()
repeat_last_sid() uses lchan->tch.las_sid.fn and lchan->tch.las_sid.buf, which is initialized from save_last_sid().
The problem is: This function is never called. Hence there is no copy of the old sid frame available, and we have no knowledge when it was sent.
DTX: fix last SID saving
Previously SID was saved explicitly by each BTS model (lc15, sysmo)
instead of relying on generic function. Fix it by using generic function
and propagating necessary parameters for it.