net: ethernet: ixp4xx: Set the DMA masks explicitly

The former fix only papered over the actual problem: the
ethernet core expects the netdev .dev member to have the
proper DMA masks set, or there will be BUG_ON() triggered
in kernel/dma/mapping.c.

Fix this by simply copying dma_mask and dma_mask_coherent
from the parent device.

Fixes: e45d0fad ("net: ethernet: ixp4xx: Use parent dev for DMA pool")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>