pandad: app_path -> app_fn (#28253)

* Fix pandad.py

Should be `app_fn` instead of `app_path`?

* fw path

---------

Co-authored-by: Adeeb Shihadeh <adeebshihadeh@gmail.com>

panda

@@ -1 +1 @@
-Subproject commit 4dd2735e38b63940ca1e1f42b4522ca4c1a0a1cc
+Subproject commit 2a5058d858b36ff7dc5477119a54f0aab137b266

selfdrive/boardd/pandad.py

@@ -7,7 +7,7 @@ import subprocess
 from typing import List, NoReturn
 from functools import cmp_to_key
 
-from panda import Panda, PandaDFU
+from panda import Panda, PandaDFU, FW_PATH
 from common.basedir import BASEDIR
 from common.params import Params
 from system.hardware import HARDWARE
@@ -16,7 +16,8 @@ from system.swaglog import cloudlog
 
 def get_expected_signature(panda: Panda) -> bytes:
   try:
-    return Panda.get_signature_from_firmware(panda.get_mcu_type().config.app_path)
+    fn = os.path.join(FW_PATH, panda.get_mcu_type().config.app_fn)
+    return Panda.get_signature_from_firmware(fn)
   except Exception:
     cloudlog.exception("Error computing expected signature")
     return b""