common: stop buffering UL measurements when SACCH is deactivated
When the BSC sends RSL DEACT SACCH, the per-SACCH UL measurement drain stops (it runs on SACCH timing), but the producer in lchan_new_ul_meas() keeps appending the measurement contributions from every received TCH/SDCCH burst. After one SACCH period (104 frames) the 104-slot uplink measurement buffer fills up, yielding a flood of:
NOTICE measurement.c:336 no space for uplink measurement, num_ul_meas=104
Add a bool sacch_active flag to gsm_lchan, set to true in the common l1sap_chan_act() and clear in l1sap_chan_deact_sacch(). Guard lchan_new_ul_meas() with this flag so that measurements are silently discarded while SACCH is inactive - there is nothing to drain the buffer and no SACCH channel on which to report the results to the BSC.
