net/au1000-eth: stop using DMA_ATTR_NON_CONSISTENT

The au1000-eth driver contains none of the manual cache synchronization
required for using DMA_ATTR_NON_CONSISTENT.  From what I can tell it
can be used on both dma coherent and non-coherent DMA platforms, but
I suspect it has been buggy on the non-coherent platforms all along.

Signed-off-by: Christoph Hellwig <hch@lst.de>