fix: correct assignment of single bit read-only storage-less fields

[calebfletcher]

Description of change

Fixes #32

I'm unsure if the condition that the branch is now checking is quite sufficient, so if there is any changes you would like to it please let me know. My understanding of the systemrdl spec is that there is only 3 different ways a field can be implemented, either as a storage element, as continuously assigned from hardware, or a constant. This branch should now run for both the storage element and the constant cases here, with the continuously assigned case already handled in the branch above it.

Checklist

• I have reviewed this project's contribution guidelines
• This change has been tested and does not break any of the existing unit tests. (if unable to run the tests, let us know)
• If this change adds new features, I have added new unit tests that cover them.

[darsor]

Thanks for the PR. I changed the logic a bit to handle the constant case a little more explicitly, then updated the unit tests to ensure single-bit fields work for all field types (combinations of hw/sw access attributes).