iommu/arm-smmu: Fix write to GERRORN register

When acknowledging global errors, the GERRORN register should be written
with the original GERROR value so that active errors are toggled.

This patch fixed the driver to write the original GERROR value to
GERRORN, instead of an active error mask.

Signed-off-by: Prem Mallappa <pmallapp@broadcom.com>
[will: reworked use of active bits and fixed commit log]
Signed-off-by: Will Deacon <will.deacon@arm.com>