When processing UL RRC messages from PDCP, the DU association ID (assoc_id)
was not being propagated to RRC, resulting in assoc_id being 0 in
rrc_gNB_decode_dcch.
Fix by setting originInstance in the ITTI message header to the stored
du_assoc_id from ue_data when PDCP creates the F1AP_UL_RRC_MESSAGE.