Full-speed isochronous transfers support with opportunistic micro-frames

Full-speed isochronous transfers support with opportunistic micro-frames
scheduling.

More work is required to properly budget and schedule micro-frames, most
of it at the HUB level.  But this let people use USB1.1 uaudio(4) devices
on ehci(4)-only systems.

Thanks to all the testers!

ok ratchov@, sthen@, naddy@