iommu/arm-smmu: Return 0 on unmap failure

The IOMMU core expects the unmap operation to return the number of bytes
that have been unmapped or 0 on failure, a negative return value being
treated like a number of bytes.

Change-Id: I76f10f7a78c779fe38199e583ddaf875752f3eb8
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: 16c50dcfc4c186ed09a4d80fbd511492d024a1c5
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>