- Remove calloc from convert_csirs_pdu function, change return value
  from 'nfapi_nr_dl_tti_csi_rs_pdu_rel15_t *' to
  'nfapi_nr_dl_tti_csi_rs_pdu_rel15_t' to not require call to 'free()'
- Set parameter pointer as const as it's not modified
- Remove cast to fapi_nr_dl_config_csirs_pdu_rel15_t in csi_rx.c as
  ue->csirs_vars[gNB_id]->csirs_config_pdu is already of type
  fapi_nr_dl_config_csirs_pdu_rel15_t.
- Remove casted parameter in call to convert_csirs_pdu and replace with
  csirs_config_pdu obtained earlier from
  ue->csirs_vars[gNB_id]->csirs_config_pdu.