iommu: arm-smmu: Handle return of iommu_device_register.

iommu_device_register returns an error code and, although it currently
never fails, we should check its return value anyway.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
[will: adjusted to follow arm-smmu.c]
Signed-off-by: Will Deacon <will.deacon@arm.com>