replace embassy_usb_driver import with re-export from embassy_usb

[dvdsk]

Most embassy users use the git version by pathing crates-io. Its easy to forget to patch both embassy-usb and embassy-usb-driver. By relying on embassy-usb-driver as re-exported by embassy-usb that can no longer happen.

[jamesmunns]

I'm not certain I want to merge this. I think the intent of relying on e-u-driver is to have fewer breaking changes as that crate changes less often than e-u does.

That being said, I wasn't able to make that work for the 0.2 to 0.3 transition, so maybe there isn't much value to that.

[dvdsk]

I am fine with leaving this open for a while (be that months or years) or closing it. Its a small change it does not matter to me who makes it if its made at all 👍. Do whatever is most convenient for you, and thanks for making postcard & postcard-rpc. They are both very nice (love the inclusion of the CobsAccumulator btw that saved me quite some time).