mv643xx_eth: only print banner once

When there are multiple mv643xx_eth silicon blocks in the system,
don't print an initialisation message for each and every one of
them.

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