Modify the return value of the uhci/ehci/xhci PCI probe routines to
'BUS_PROBE_DEFAULT'. This allows bhyve's 'ppt' driver to claim ownership
of the device and pass it through to the guest.
In the common case where there are no competing drivers for USB controllers
this change is a no-op.
Reviewed by: hselasky
MFC after: 2 weeks