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.

MFC after:	3 days
When we fail to get a USB reference we should just return, because
there are no more references held.

MFC after:	3 days