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.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>