Fix/workaround to support multiple UEs

Temporarily replace assertion with warnings for issues seen with multiple UEs connecting to gNB. Two assertions:

1. caused by bug in MSG4 rtx. Fixed in MR1324
2. caused by multiple PDUs of type NFAPI_NR_DL_TTI_PDCCH_PDU_TYPE in the same slot. To be fixed in MR1359. The assertion is shown below.

        case NFAPI_NR_DL_TTI_PDCCH_PDU_TYPE:
          AssertFatal(pdcch_received == 0, "pdcch_received is not 0, we can only handle one PDCCH PDU per slot\n");
          msgTx->pdcch_pdu = dl_tti_pdu->pdcch_pdu;
          pdcch_received = 1;
          break;

The symptom is that one UE is under DL iperf and the other UE is doing RA. Is it possible that this code does not handle multiple UEs? As shown below in the figure, there are multiple places that create PDU of type NFAPI_NR_DL_TTI_PDCCH_PDU_TYPE