When a device has a DMA offset the dma capable result will change due
to the difference between the physical and DMA address.  Take that into
account.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: NRobin Murphy <robin.murphy@arm.com>