SPARC64: Fix sun4v DMA panic

64bit DMA only supported on sun4v equipped with ATU IOMMU HW.
'Commit b02c2b0b

 ("sparc: remove arch specific dma_supported
implementations")' introduced a code that incorrectly allow
dma_supported() to succeed for 64bit dma mask even if system doesn't
have ATU IOMMU. This results into panic.

Fix it.

Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Tushar Dave <tushar.n.dave@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>