apb: Fix out-of-bounds array write access

The array regs is declared with IOMMU_NREGS (3) elements and accessed
using IOMMU_CTRL (0) and IOMMU_BASE (8). In most cases, those values
are right shifted before being used as an index which results in indices
0 and 1. In one case, this right shift was missing for IOMMU_BASE which
results in an out-of-bounds write access with index 8.

The patch adds the missing shift operation also for IOMMU_CTRL where
it is needed only for cosmetic reasons.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>