iommu: change iommu_map_sgtable to return signed values

iommu: change iommu_map_sgtable to return signed values

The iommu_map_sgtable() function returns ssize_t and negative error
codes but it's declared as size_t instead.  I think that static checkers
would have complained if this caused a bug, but even though it doesn't
cause a bug, it's definitely worth fixing.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/06672b96-23fd-424c-8880-1626e7bf119c@moroto.mountain
Signed-off-by: Joerg Roedel <jroedel@suse.de>