usb: add the Endpoint::is_enabled function

[mattico]

I found myself calling wait_enabled() with a timeout to try to determine if the endpoint is enabled, which is silly when we can just read the register.

[Dirbaio]

lgtm, but this is a breaking change of embassy-usb-driver which is somewhat disruptive (needs releasing updates for all of embassy-usb, all crates that use it, and all hal crates), so we should batch it together with a few other pending embassy-usb-driver improvements. there's a few issues about these.