Support enable/disable SOF interrupt in USB peripheral

[henrikssn]

@twitchyliquid64 I haven't tested it yet but is this inline with your suggestion?

[twitchyliquid64]

Yepp!

We should be careful about testing it extensively, depending on the pecking order of the interrupts we may need to clear it in other places of poll() as well.

[ianrrees]

Thanks for this! It's a really handy bit towards solving a problem I've been mulling over. What's the use case that prompted this initially?

I'm toying with a usb class implementation that is meant to do all its work in the poll() method, with an eye to making the bus own the class (per 2 here). One issue with that approach currently, is that the class doesn't have a way to notify when it wants attention from the bus.

Currently this change doesn't trigger poll() for each class, but it looks like a straightforward modification to usb-device could change that, or this could be a more permanent solution.

[jbeaurivage]

Closing, as this was superseded by #456.