rstp: Add comment in rstp_port_set_mcheck().
rstp: Edit xport_rstp_learn_state(), xport_rstp_forward_state() and
      xport_rstp_should_manage_bpdu().
rstp: Change return type to void in set_rstp(), get_rstp_status(),
      set_rstp_port() and get_rstp_port_status().