sky2: implement 64 bit stats

This implements 64 bit statistics support and fixes races when reading
counter values. The PHY counters can only be accessed 16 bits at a time,
so they are subject to carry races.

NB:
  * TX/RX counters are maintained in software because the the hardware packet count
    is only a 32 bit value.

  * Error counters are really only 32 bit.

  * Old 32 bit counter fields in dev->stats still used for some
    software counters

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>