hw/i386/intel_iommu: Simplify vtd_find_as_from_bus_num() logic

hw/i386/intel_iommu: Simplify vtd_find_as_from_bus_num() logic

The vtd_find_as_from_bus_num() function was introduced (in commit
dbaabb25f) in a code format that could return an incorrect pointer,
which was later fixed by commit a2e1cd41ccf.
We could have avoided this by writing the if() statement differently.
Do it now, in case this function is re-used. The code is easier to
review (harder to miss bugs).

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200305102702.31512-1-philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>