lib82596: convert to dma_alloc_noncoherent

Use the new non-coherent DMA API including proper ownership transfers.
This includes moving the DMA helpers to lib82596 based of an ifdef to
avoid include order problems.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de> (SNI part)