xhci(4): Defer root intr xfers while polling.

Root intr xfers require taking adaptive locks, which is forbidden
while polling.

This is not great -- any USB transfer completion callbacks might try
to take adaptive locks, not just uhub_intr, and that will always
causes trouble.  We get lucky with ukbd_intr because it's not
MP-safe, so it relies only on the kernel lock (a spin lock) anyway.
But this change brings xhci in line with ehci.

PR kern/57326

XXX pullup-8
XXX pullup-9
XXX pullup-10

(riastradh)