RemoveUselessEnums explodes with useless enum that has no `0` variant.

From the MSPM0C110x SVD files, with the SYSCTL peripheral, the following enum causes a panic in `!RemoveUselessEnums` with the latest chiptool (57953f5c)

Yes this enum is some internal testing or undefined register which TI left in their SVD files...

```yaml
enum/FLBANK_SWAP:
bit_size: 1
variants:
- name: YES
description: System has a CSC.
value: 1
- name: NO
description: No, system does not have a CSC.
value: 2
```

I suspect the issue is that `YES` has a value of 1 and `NO` has a value of 2, which causes the transform to explode since there is no variant with a value of `0`.

```
thread 'main' panicked at src/transform/delete_useless_enums.rs:76:69:
called `Option::unwrap()` on a `None` value
stack backtrace:
0: rust_begin_unwind
1: core::panicking::panic_fmt
2: core::panicking::panic
3: core::option::unwrap_failed
4: chiptool::transform::delete_useless_enums::DeleteUselessEnums::run
5: chiptool::apply_transform
6: chiptool::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
```