lchan_fsm: don't mark lchan as BORKEN on unsupported mode NACK
When the BTS NACKs a Channel Activation with RSL_ERR_SERV_OPT_UNAVAIL or RSL_ERR_SERV_OPT_UNIMPL, it means the requested service or channel mode is not supported - not that the hardware is broken. In this case the lchan should transition to LCHAN_ST_WAIT_AFTER_ERROR rather than LCHAN_ST_BORKEN, which is reserved for genuine hardware failures.
Also add LCHAN_EV_RTP_ERROR to LCHAN_ST_WAIT_AFTER_ERROR's in_event_mask so that a late LCHAN_EV_RTP_ERROR (e.g. MGCP CRCX timeout arriving after the NACK was already handled) is silently absorbed rather than triggering an unexpected-event warning.
