[ehci] Support USB1 devices attached via transaction translators

Support low-speed and full-speed devices attached to a USB2 hub.  Such
devices use a transaction translator (TT) within the USB2 hub, which
asynchronously initiates transactions on the lower-speed bus and
returns the result via a split completion on the high-speed bus.

We make the simplifying assumption that there will never be more than
sixteen active interrupt endpoints behind a single transaction
translator; this assumption allows us to schedule all periodic start
splits in microframe 0 and all periodic split completions in
microframes 2 and 3.  (We do not handle isochronous endpoints.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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