forcedeth: remove unneeded stats updates

Function ndo_get_stats() updates most of the stats from hardware
registers, making the manual updates un-needed. This change removes
these manual updates. Main exception is rx_missed_errors which needs
manual update.

Another exception is rx_packets, still updated manually in this commit
to make sure this patch doesn't change behavior of driver. This will
be addressed by a future patch.

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