Don't do OFDM mod on PRACH slots

set number of prach slots in prach_vars in first slot and set as
inactive in last slot so we can skip OFDM modulation in slots that
already has prach.

openair1/PHY/defs_nr_UE.h

@@ -298,6 +298,7 @@ typedef struct {
 typedef struct {
   int16_t amp;
   bool active;
+  int num_prach_slots;
   fapi_nr_ul_config_prach_pdu prach_pdu;
 } NR_UE_PRACH;