usb-queue: Add flush utility
usb_standard_request: Always call configuration_changed_cb

For reasons I don't entirely understand, bulk requests are suddenly
ignored after a SET_CONFIGURATION request (even if the configuration did
not change) unless the endpoints are reinitialized. This is done by
configuration_changed_cb, therefore we call it for every request.
uadfa
usb: Ensure endpoint queue is flushed on disable/init