amd_iommu: Fix the calculation for Device Table size

amd_iommu: Fix the calculation for Device Table size

Correctly calculate the Device Table size using the format encoded in the
Device Table Base Address Register (MMIO Offset 0000h).

Cc: qemu-stable@nongnu.org
Fixes: d29a09ca6842 ("hw/i386: Introduce AMD IOMMU")
Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Message-Id: <20250617150427.20585-7-alejandro.j.jimenez@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>