I made a bug in can unpack maybe its fixed now oops (#1316)

### Changelist 
<!-- Give a list of the changes covered in this PR. This will help both
you and the reviewer keep this PR within scope. -->

### Testing Done
<!-- Outline the testing that was done to demonstrate the changes are
solid. This could be unit tests, integration tests, testing on the car,
etc. Include relevant code snippets, screenshots, etc as needed. -->

### Resolved Tickets
<!-- Link any tickets that this PR resolves. -->

scripts/code_generation/jsoncan/src/can_database.py

@@ -328,6 +328,7 @@ def unpack(self, id: int, data: bytes) -> Dict:
                 if signal_bits & (1 << (signal.bits - 1)):
                     signal_bits = ~signal_bits & ((1 << signal.bits) - 1)
                     signal_bits += 1
+                    signal_bits *= -1
 
             # Decode the signal value using the scale/offset.
             signal_value = signal_bits * signal.scale + signal.offset