arm64: dma-mapping: use correct type for iova in arm_iommu_unmap_sg

IOMMU virtual addresses use the dma_addr_t type since they can be up to
64-bits.  We're currently using an `unsigned int' to store our IOVA in
arm_iommu_unmap_sg, which could result in truncation.  Use the correct
type for an I/O virtual address: dma_addr_t.

This was previously fixed for arm_iommu_map_sg in
[02454d7f9feeb: "arm64: dma-mapping: use correct type for iova"].

Make the same fix in arm_iommu_unmap_sg.

Change-Id: Ib22a9600f33e6fa155812b08d67d62f72af0ad8e
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>