iommu/amd: Fix variable "iommu" set but not used

iommu/amd: Fix variable "iommu" set but not used

The commit dce8d6964ebd ("iommu/amd: Convert to probe/release_device()
call-backs") introduced an unused variable,

drivers/iommu/amd_iommu.c: In function 'amd_iommu_uninit_device':
drivers/iommu/amd_iommu.c:422:20: warning: variable 'iommu' set but not
used [-Wunused-but-set-variable]
  struct amd_iommu *iommu;
                    ^~~~~

Signed-off-by: Qian Cai <cai@lca.pw>
Link: https://lore.kernel.org/r/20200509015645.3236-1-cai@lca.pw
Fixes: dce8d6964ebd ("iommu/amd: Convert to probe/release_device() call-backs")
Signed-off-by: Joerg Roedel <jroedel@suse.de>