iommu/arm-smmu: Fix the error path in arm_smmu_add_device

fwspec->iommu_priv is available only after arm_smmu_master_cfg
instance has been allocated. We shouldn't free it before that.
Also it's logical to free the master cfg itself without
checking for fwspec.

Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
[will: remove redundant assignment to fwspec]
Signed-off-by: Will Deacon <will.deacon@arm.com>