iommu/arm-smmu: Fix uninitilized variable warning

Some unrelated changes in the iommu code caused a new warning to
appear in the arm-smmu driver:

  CC      drivers/iommu/arm-smmu.o
drivers/iommu/arm-smmu.c: In function 'arm_smmu_add_device':
drivers/iommu/arm-smmu.c:1441:2: warning: 'smmu' may be used uninitialized in this function [-Wmaybe-uninitialized]
  arm_smmu_rpm_put(smmu);
  ^~~~~~~~~~~~~~~~~~~~~~

The warning is a false positive, but initialize the variable to NULL
to get rid of it.
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Will Deacon <will@kernel.org> # arm-smmu
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/r/20200326150841.10083-8-joro@8bytes.org

drivers/iommu/arm-smmu.c

@@ -1383,7 +1383,7 @@ struct arm_smmu_device *arm_smmu_get_by_fwnode(struct fwnode_handle *fwnode)
 
 static int arm_smmu_add_device(struct device *dev)
 {
-	struct arm_smmu_device *smmu;
+	struct arm_smmu_device *smmu = NULL;
 	struct arm_smmu_master_cfg *cfg;
 	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
 	int i, ret;