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.

[xiaguangbo]

1bit only 0 or 1, no 2. manully fix svd

[i509VCB]

1bit only 0 or 1, no 2. manully fix svd

Yes the SVD files are wrong with regards to valid values. At the same time this is an undocumented register bit. How can I know that 0 and 1 are indeed the correct values?

I currently work around this by just removing the enum since it's "undefined" per reference manuals