ARM: dma-mapping: Set proper DMA ops in arm_iommu_detach_device()

Instead of setting the DMA ops pointer to NULL, set the correct,
non-IOMMU ops depending on the device's coherency setting.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>