iommu/tegra: gart: Fix gart_iommu_unmap()

It must return the number of unmapped bytes on success, returning 0 means
that unmapping failed and in result only one page is unmapped.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>