[usb] ehci/hubs - implementation of TT

[Meulengracht]

EHCI
Start-split transactions initiates low/full-speed transfer through an high-speed hub. Complete-split act kind of like get-status of the start-split transaction, and is something we must poll for after an appropriate time. I'm assuming we are able to calculate the expected transfer delay based on type/size. We won't get an interrupt from the hub when a transfer is done.

• Implementation of Transaction start/split transfers when target is a hub and device low or full-speed.
• Verify that we send the correct TT address in the usb transfer

Hub:
Hubs can have one or multiple TT buffers, this should be reported in the class protocol. E.g one total or one per port.

• Implementation of TT_* functions