scripts: gen_handles.py: initialize handle.device

Initialize `handle.device` to `None` so that the assert after the
matching has a chance to catch errors. Without this, a failed match will
raise an exception on the line above as it attempts to get a property
that doesn't exist.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>

scripts/gen_handles.py

@@ -211,6 +211,7 @@ def main():
 
     used_nodes = set()
     for handle in handles:
+        handle.device = None
         for device in devices:
             if handle.addr == device.obj_handles:
                 handle.device = device