[PATCH] mv643xx_eth: Fix ethtool stats

Some stats reported by ethtool -S on mv643xx_eth device are cleared
between each call.  This patch fixes it.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>