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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272479 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f