bugfix: fix timing advance

- re-use ta_timer
- when ta_timer is running:
  - no TA sent to UE
  - no TA from UE (for UE to have time to apply previous TA command)
- decrease ta_timer every TTI
- be careful to use correct value of ta_update (31 means no TA)

Maybe not correct, to be checked.

openair2/LAYER2/MAC/pre_processor.c

@@ -894,6 +894,8 @@ void dlsch_scheduler_pre_processor_reset (int module_idP,
   LOG_D(MAC,"Running preprocessor for UE %d (%x)\n",UE_id,rnti);
   // initialize harq_pid and round
 
+  if (ue_sched_ctl->ta_timer) ue_sched_ctl->ta_timer--;
+
   /*
   eNB_UE_stats *eNB_UE_stats;