iommu/arm-smmu: Tighten up global fault reporting

On systems which use a single, combined irq line for the SMMU, context
faults may result in us spuriously reporting global faults with zero
status registers.

This patch fixes up the fsr checks in both the context and global fault
interrupt handlers, so that we only report the fault if the fsr
indicates something did indeed go awry.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>