TSCH: declare `radio_rx_mode` only when HW timestamps are enabled to avoid warnings
TSCH: declare `radio_rx_mode` only when HW timestamps are enabled to avoid warnings

MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki