RemoveUselessEnums explodes with useless enum that has no 0 variant.

[i509VCB]

From the MSPM0C110x SVD files, with the SYSCTL peripheral, the following enum causes a panic in !RemoveUselessEnums with the latest chiptool (57953f5)

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

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.