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

drivers/iommu/arm-smmu-v3.c:2989:26:
warning: variable ‘smmu’ set but not used [-Wunused-but-set-variable]
  struct arm_smmu_device *smmu;
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NChen Zhou <chenzhou10@huawei.com>
Link: https://lore.kernel.org/r/20200508014955.87630-1-chenzhou10@huawei.comSigned-off-by: NWill Deacon <will@kernel.org>