Fixes gcc '-Wunused-but-set-variable' warning:

drivers/iommu/amd_iommu.c: In function 'iommu_print_event':
drivers/iommu/amd_iommu.c:550:33: warning:
 variable 'tag' set but not used [-Wunused-but-set-variable]

It was introduced in
  e7f63ffc ("iommu/amd: Update logging information for new event type")

seems just missed in the error message, add it as suggested by Joerg.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>