hw/arm/virt-acpi-build: Drop local iort_node_offset

hw/arm/virt-acpi-build: Drop local iort_node_offset

Both the other two callers of build_iort_id_mapping() just directly pass
in the IORT_NODE_OFFSET macro. Keeping a "const uint32_t" local variable
storing the same value doesn't have any gain.

Simplify this by replacing the only place using this local variable with
the macro directly.

Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Message-Id: <20240619001708.926511-1-nicolinc@nvidia.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>