AMD IOMMU: don't BUG() when we don't have to

find_iommu_for_device() can easily return NULL instead, as all of its
callers are prepared for that.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
+++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
@@ -32,8 +32,8 @@ struct amd_iommu *find_iommu_for_device(
 {
     struct ivrs_mappings *ivrs_mappings = get_ivrs_mappings(seg);
 
-    BUG_ON ( bdf >= ivrs_bdf_entries );
-    return ivrs_mappings ? ivrs_mappings[bdf].iommu : NULL;
+    return ivrs_mappings && bdf < ivrs_bdf_entries ? ivrs_mappings[bdf].iommu
+                                                   : NULL;
 }
 
 /*