iommu/arm-smmu: fix issue by devm_free_irq on cb interrupt line

Fix the issue caused by the devm_free_irq on a unfilled CB interrupt
array by filling the cfg->irptndx at proper place.

devm_free_irq+0x58/0x60
arm_smmu_destroy_domain_context+0xc4/0x188
arm_smmu_attach_dev+0xb00/0xb2c
__iommu_attach_device+0x4c/0x110
__iommu_attach_group+0x54/0x74
iommu_attach_group+0x30/0x50
arm_iommu_attach_device+0x88/0x2f

Change-Id: I50cb4206ab56d78809777543162edf68974fcc37
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>