Changes

osmo-bts-trx: trx_fn_timer_cb(): fix misleading shutdown reason

If osmo-bts-trx exit()s due to the PC clock issues, e.g. if the
process stalls, it produces rather confusing logging messages:

  DL1C ERROR PC clock skew: elapsed_us=387574, error_us=382959
  DOML NOTICE ... Shutting down BTS, exit 1, reason: No clock from osmo-trx

The second message suggests that the transceiver (osmo-trx) is the
culprit, but the first one reflects the actual reason (PC clock skew).

Let's pass proper shutdown reason to avoid confusion.

Change-Id: Ibbbbc4e919e6eb812882fc60de4be13fa77934b7