PCIe allows for MSI-X BAR to be either dedicated, or MSI-X Table may be co-located in some functional BAR. In the later case xhci(4) is unable to allocate active resource for the table because BAR is already activated.
Handle it by checking for this special case, and not try to alloc resource if MSI-X BAR is IO.
Reported and tested by: emaste
Note. It seems that a similar issue exists in iflib.