iommu/amd: Fix alloc_irq_index() increment

On an is_allocated() interrupt index, we ALIGN() the current index and
then increment it via the for loop, guaranteeing that it is no longer
aligned for alignments >1.  We instead need to align the next index,
to guarantee forward progress, moving the increment-only to the case
where the index was found to be unallocated.

Fixes: 37946d95 ('iommu/amd: Add align parameter to alloc_irq_index()')
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>