mv643xx_eth: identify ports by struct mv643xx_private *

Instead of identifying individual mv643xx ethernet ports by only
their port number, identify them by their struct mv643xx_private *,
as just a port number has no meaning when there are multiple
mv643xx_eth silicon blocks in the system.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Reviewed-by: Tzachi Perelstein <tzachi@marvell.com>
Signed-off-by: Dale Farnsworth <dale@farnsworth.org>