iommu/arm-smmu: Fix uninitialized use of variable error

There are couple of instances where we are declaring
variable without proper initialization and we
might end up returning these uninitialized
variables. Fix this by initializing them during
declaration.

Change-Id: Icd5b504c35683e94383d2cefd186f66000478e82
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>