[netdrvr 8139too] make features more persistent; fix PCI DAC mode

[netdrvr 8139too] make features more persistent; fix PCI DAC mode

* only set PCIDAC (64-bit PCI) bit in hardware if
  sizeof(dma_addr_t) > 32.  Need a better test for whether
  64-bit mode is _really_ needed.
* cache chip command register in private struct.  this allows
  the setting of rx-vlan, rx-csum, and other features to be
  persistent across the entire lifetime of the net device.
* remove dead private struct members frag_skb, dropping_frag,
  and pci_using_dac.