making timing advance a int (instead of uint)

targets/RT/USER/lte-softmodem.c

@@ -320,7 +320,7 @@ static LTE_DL_FRAME_PARMS      *frame_parms[MAX_NUM_CCs];
 int multi_thread=1;
 uint32_t target_dl_mcs = 28; //maximum allowed mcs
 uint32_t target_ul_mcs = 10;
-uint32_t timing_advance = 0;
+int32_t timing_advance = 0;
 uint8_t exit_missed_slots=1;
 uint64_t num_missed_slots=0; // counter for the number of missed slots