MFC r272349, r272422 and r272479:
- Fix XHCI driver for devices which have more than 15 physical root HUB
  ports. The current bitmap array was too small to hold more than 16
  bits and would at some point toggle the context size, which then would
  trigger an enumeration fault and cause a fallback to the EHCI
  companion controller, if any.
- Make sure we always set the maximum number of valid contexts.
- Set default cycle state in case of early interrupts.
MFC r272480:
When we fail to get a USB reference we should just return, because
there are no more references held.