[USB] ehci-hcd detects microframe transaction scheduling collisions

This detects TT schedule collisions for split periodic transactions
on USB 2.0 hubs, handling both isochronous and interrupt transfers
but not supporting the "wrap around to the next frame" modes for
either transfer type.

The ISO scheduler is modified to use that support for full speed
transfers ... though nothing will yet ask it to do that.

It also tracks schedule depth, to help avoid accepting ISO URBs
which need more time than currently remains unscheduled.