ixp4xx_eth: avoid calling dma_pool_create() with NULL dev

Use &port->netdev->dev instead of NULL since dma_pool_create() doesn't
allow NULL dev.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>