mv643xx_eth: kill private unused instance of struct net_device_stats

The per-port mv643xx_eth_private struct had a private instance
of struct net_device_stats that was never ever written to, only
read (via the ethtool statistics interface).  This patch gets
rid of the private instance, and tweaks the ethtool statistics
code in mv643xx_eth to use the statistics in struct net_device
instead.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>