For multiple UEs, we need to be able to filter the rx'd messages by
the RNTI. However, we do not have the RNTI value until the CFRA (NSA)
or CBRA (SA) procedure is complete. The check below will handle this.
Also, depending on the RNTI value, we can have a SIB (0xffff), RAR (0x10b),
Msg3 (TC_RNTI) or an actual DCI message (CRNTI). When we get Msg3, the
MAC instance of the UE still has a CRNTI = 0. Only once the RA procedure
succeeds is the CRNTI value updated to the TC_RNTI.