xen/pci: address a violation of MISRA C Rule 16.3

Refactor the code to avoid an implicit fallthrough and address
a violation of MISRA C:2012 Rule 16.3: "An unconditional `break'
statement shall terminate every switch-clause".

No functional change.

Signed-off-by: Federico Serafini <federico.serafini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/drivers/passthrough/pci.c

@@ -170,8 +170,11 @@ static int __init cf_check parse_phantom_dev(const char *str)
     {
     case 1: case 2: case 4:
         if ( *s )
-    default:
             return -EINVAL;
+        break;
+
+    default:
+        return -EINVAL;
     }
 
     phantom_devs[nr_phantom_devs++] = phantom;