USB: Fix parsing of device files

[duckfullstop]

Turns out I wrote this completely wrong 🙃

This also adds a little extra logging for when things go wrong, as well as tweaking some of the codepaths to hopefully make testing a little easier (something I'd like to add when I get some spare time).

[duckfullstop]

Oh geez Git threw an absolute wobbler with the merge commit here - not sure if you want to merge this or whether I should attempt to unmangle it with some force pushing?

[squat]

We can squash and merge and it's like nothing happened, it seems like only the very last commit is significant here, so luckily reviewing isn't difficult