iommu/vt-d: Silence a variable set but not used

The commit "iommu/vt-d: Probe DMA-capable ACPI name space devices"
introduced a compilation warning due to the "iommu" variable in
for_each_active_iommu() but never used the for each element, i.e,
"drhd->iommu".

drivers/iommu/intel-iommu.c: In function 'probe_acpi_namespace_devices':
drivers/iommu/intel-iommu.c:4639:22: warning: variable 'iommu' set but
not used [-Wunused-but-set-variable]
  struct intel_iommu *iommu;

Silence the warning the same way as in the commit d3ed71e5
("drivers/iommu/intel-iommu.c: fix variable 'iommu' set but not used")

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Joerg Roedel <jroedel@suse.de>