lib82596: move DMA allocation into the callers of i82596_probe

This allows us to get rid of the LIB82596_DMA_ATTR defined and prepare
for untangling the coherent vs non-coherent DMA allocation API.

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