GM: add checksum to button message (#26014)

* Add GM button msg checksum

* checksum that works for both, fix missing bit

* bump opendbc

* update comment

Co-authored-by: Shane Smiskol <shane@smiskol.com>

opendbc

@@ -1 +1 @@
-Subproject commit 1f8aa057bc1c96fcf8a2b612a9897ce91e627381
+Subproject commit bb7182902847b84aa572733edfd8e7073bb02aeb

selfdrive/car/gm/gmcan.py

@@ -6,7 +6,16 @@ def create_buttons(packer, bus, idx, button):
   values = {
     "ACCButtons": button,
     "RollingCounter": idx,
+    "ACCAlwaysOne": 1,
+    "DistanceButton": 0,
   }
+
+  checksum = 240 + int(values["ACCAlwaysOne"] * 0xf)
+  checksum += values["RollingCounter"] * (0x4ef if values["ACCAlwaysOne"] != 0 else 0x3f0)
+  checksum -= int(values["ACCButtons"] - 1) << 4  # not correct if value is 0
+  checksum -= 2 * values["DistanceButton"]
+
+  values["SteeringButtonChecksum"] = checksum
   return packer.make_can_msg("ASCMSteeringButton", bus, values)