This commit enables UL traffic in do-ra mode by doing two changes:
- configure SDAP to send traffic to TUN interface instead of GTP by
  abusing enb_flag=0 (i.e., setting UE mode for gNB SDAP)
- manually enable RLC LCID in UE MAC, since nr_DRB_preconfiguration()
  statically enables one DRB bearer, without notifying MAC