osmo-bts-trx: shut down on stale clock indication from transceiver
We expect the transceiver to be a reliable, monotonic clock source. If it reports an FN far behind our local timer (elapsed_fn < 0) while far more wall-clock time elapsed than its FN advance accounts for, its clock has likely stalled and the indication carries a stale frame number. Acting on it drags the scheduler backwards and re-transmits already-sent TDMA frames, corrupting lchan-internal state(s).
Detect this and shut down the process, same rationale as the existing "PC clock skew too high" check in trx_fn_timer_cb().
