Handle copybreak.
- velocity_rx_refill() is modified to allow the processing of a Rx desc
  ring wherein the empty skb slots are not necessarily contiguous. Given
  the preceeding changes, rx_copybreak should not need anything else;
- the driver does not rely on rd_info->skb_dma set to NULL any more;
- pci_dma_sync_single_for_{cpu/device} changes as a bonus;
- more function documentation.

Some inspiration borrowed from similar r8169 code.