These changes add limited polled mode support to the XHCI driver:

- only input is supported

- only USB interrupt transfers are supported
This is good enough to get a USB keyboard working to be able to use the kernel debugger.

I've tested this on a Celistica Athena. Without my change "mdb -K" on the IPMI remote console will result in a hung system.
With my change the kernel debugger prompt appears and I'm able to request a stack trace with "$C" and exit the debugger with ":c".