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