iommu/tegra-gart: Remove direct access of dev->iommu_fwspec

Use the accessor functions instead of directly dereferencing
dev->iommu_fwspec.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/r/20200326150841.10083-5-joro@8bytes.org