- DEBUG_PARANOIA was bad. "if (debug > x) {" is nice;
- state_check() now has only one return point;
- try_get_rx_skb() cosmetic;
- dscc4_rx_update() belongs to HOLD mode to LxDA changes;
- dscc4_wait_ack_cec() behaves like dscc4_xpr_ack();
- dscc4_rx_skb() refill logic is ready for LxDA mode and does everything
  to fulfill what its name suggests.
- document some errata voodoo in dscc4_init_one();
- dscc4_init_ring() should handle try_get_rx_skb() failure.