net: ethernet: ixp4xx: Use parent dev for DMA pool

Use the netdevice struct device .parent field when calling
dma_pool_create(): the .dma_coherent_mask and .dma_mask
pertains to the bus device on the hardware (platform)
bus in this case, not the struct device inside the network
device. This makes the pool allocation work.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>