amd_iommu: Fix truncation of oldval in amdvi_writeq

amd_iommu: Fix truncation of oldval in amdvi_writeq

The variable `oldval` was incorrectly declared as a 32-bit `uint32_t`.
This could lead to truncation and incorrect behavior where the upper
read-only 32 bits are significant.

Fix the type of `oldval` to match the return type of `ldq_le_p()`.

Cc: qemu-stable@nongnu.org
Fixes: d29a09ca6842 ("hw/i386: Introduce AMD IOMMU")
Signed-off-by: Ethan Milon <ethan.milon@eviden.com>
Message-Id: <20250617150427.20585-9-alejandro.j.jimenez@oracle.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>