Missing pieces for DMA-API on the Rx side:
- SiS190_init_ring: the global area for the received data is mapped.
  This area is persistent during the whole driver's life.
  It only needs to be unmapped in SiS190_close() as no other exit/error
  path exists.
- SiS190_rx_interrupt: no map/unmap for received data buffer. A single
  sync operation is done. Btw, there is no need to store the same value
  in RxDescArray[cur_rx].buf_addr over and over again.
- Remove driver dependancy on CONFIG_BROKEN.