mv643xx_eth: Fix MV643XX_ETH offsets used by Pegasos 2

In the mv643xx_eth driver, we now use offsets from the ethernet
register block within the chip, but the pegasos 2 platform still
needs offsets from the full chip's register base address.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>