forcedeth: fix stats on hardware without extended stats support

This change makes sure that tx_packets/rx_bytes ifconfig counters are
updated even on NICs that don't provide hardware support for these
stats: they are now updated in software. For the sake of consistency,
we also now have tx_bytes updated in software (hardware counters
include ethernet CRC, and software doesn't account for it).

This reverts parts of:
 - "forcedeth: statistics optimization" (21828163)
 - "forcedeth: Improve stats counters" (0bdfea8b)
 - "forcedeth: remove unneeded stats updates" (4687f3f3

)

Tested:
  pktgen + loopback (http://patchwork.ozlabs.org/patch/124698/)
  reports identical tx_packets/rx_packets and tx_bytes/rx_bytes.

Signed-off-by: David Decotigny <david.decotigny@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>