iommu/amd: Fix error code path in early_amd_iommu_init()

Prevent early_amd_iommu_init() from leaking memory mapped via
acpi_get_table() if check_ivrs_checksum() returns an error.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>