11281 XHCI polled mode support for USB keyboards

Review Request #2028 — Created June 28, 2019 and submitted — Latest diff uploaded


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".