UsbBus::poll disables all interrupts

[dimpolo]

I didn't see this documented and was quite surprised by this.

Is the reason simply because target::interrupt::Mutex requires a CriticalSection or are there hardware reasons for disableing interrupts? Could the Mutex be replaced by a RefCell? Edit: UsbBus: Sync ... so no :(

(My use case is an ADC ISR that must not be disabled)