sparc64/iommu: allow large DMA masks

We've been moving to a model where the device just sets the DMA mask
supported by it, instead of having to fallback to something it thinks
the platform might support.  Sparc64 is the remaining holdout forcing
drivers to supply a matching mask.  Change dma_4u_supported to just
check if the supplied dma mask is large enough as nothing in the
iommu.c code (or the core DMA code) actually looks at the DMA mask
later on.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: David S. Miller <davem@davemloft.net>