[netdrvr] forcedeth: internal simplifications; changelog removal

* remove changelog from source; its kept in git repository

* consolidate descriptor version tests using nv_optimized()

* consolidate NIC DMA start, stop and drain into
  nv_start_txrx(), nv_stop_txrx(), nv_drain_txrx()

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>