Sync with Intel's em-6.2.9

Sync with Intel's em-6.2.9
- Support more chips.
- Implement suspend/resume device methods.
- Correct promisc mode support.
- Code and comment clean up.
- Avoid reinitializing the hardware if only IP address is changed.  This is
  implemented in a different way from Intel's:
  If IFF_RUNNING is set, em_init() will return immediately.  So for some
  situation in which em_init() must run, IFF_RUNNING is cleared before calling
  em_init().
- Rework TX processing:
  In em_encap(), save the index of the packet's last TX descriptor (EOP) in
  the packet's first descriptor.  In em_txeof(), test E1000_TXD_STAT_DD bit
  only for last TX descriptor (EOP) of certain packet.  If it is set, then
  reap the TX descriptors of the packet in an inner loop.
  "This change is important for future chips, where DD bit is going
   to be set only on the EOP descriptors." -- Jack Vogel at Intel