[netdrvr sis190] convert TX path to use PCI DMA API

[netdrvr sis190] convert TX path to use PCI DMA API

Also, minor changes:
* mark ->hard_start_xmit ETH_ZLEN test as unlikely()
* use cpu_to_le32() and le32_to_cpu() in TX path
* fix two leak in error path, in ->hard_start_xmit
* don't test netif_queue_stopped() in TX completion path,
  netif_wake_queue() already does that.