freebsd stable - Aug 2012 - uhci0 excessive interrupts---how can I disable or reset specific USB port?

I am running FreeBSD/amd64 9-STABLE (built Mon Jul 23 10:45:51 EDT 2012) on a
Dell Optiplex 760 and, today, noticed I had almost 30% system CPU load in top
even when the system was idle.  A perusal of vmstat revealed the cause to be
excessive interrupts on uhci0, even though nothing was plugged into that USB
port:

# vmstat -i
interrupt                          total       rate
irq4: uart0                           22          0
irq16: uhci0                617002282738     310969
irq23: uhci3 ehci1                    83          0
irq256: hpet0:t0               135818421         68
irq257: hpet0:t1              2222659301       1120
irq264: em0                     29529304         14
irq265: ahci0                   11132506          5
Total                       619401422375     312178


Because I am only using the front USB ports on that hardware, I thought I would
disable the other (rear) USB ports in the BIOS.  I rebooted and duly disabled
them.  However, when FreeBSD booted, it appeared to ignore the BIOS setting: all
the USB ports were probed as usual.  (The high interrupts had vanished, though
that might have been due to FreeBSD correctly shutting down the controllers at
shutdown, or just the act of rebooting itself.)

I added "hint.uhci.0.disabled=1" to /boot/loader.conf (hoping it would
disable uhci0), but, again all the USB ports appeared in the boot probes. 
(However, it *appears* as if uhci0 has been disabled because the "irq16:
uhci0" line no longer appears in "vmstat -i".  However, all the
same ugen devices appear in /dev.)

Is there a way of disabling a specific USB controller that you don't want to
use?  If so, how?  I looked at usbconfig, but that appears to me to be more
about controlling devices plugged in to a USB port rather than the port itself. 
The "reset" command of usbconfig appears to be about resetting USB
devices, not ports.

If I can't disable a specific USB port, is there a way to reset it without
rebooting?  If I ever get another crazy interrupt storm like I noticed today it
would be nice to be able to stop it without having to do a reboot.

Cheers,

Paul.