O(/E)HCI detach support

[Craig Boston]

Okay, sorry for the reply-to-self, but I think I've nailed down the detach 
problem and it looks like it will require some more work.  In short, the OHCI 
driver may be ready for hot-plugging but the USB subsystem isn't.  Here's a 
play-by-play of what's happening:

1. On card removal, cardbus_detach_card dispatches ohci_pci_detach
2. ohci_pci_detach calls device_delete_child on itself
3. device_delete_child calls itself recursively for the children of ohci
4. When device_delete_child tries to dispatch device_detach for the first uhub 
it finds, it gets ENXIO because uhub doesn't define a detach method
5. The ENXIO error filters up to the top level device_delete_child and it 
exits without detaching any of the other devices
6. ohci_pci_detach ignores the error result and removes all its resources 
(about all it can do at this point)
7. cardbus_detach_card removes the PCI configuration data from memory

This leaves some nonexistent devices as children of the cardbus device.  The 
next time any card is inserted, it calls cardbus_detach_card to remove the 
old entries, which removes the already-removed PCI config, and BOOM.  I think 
the only way this can be resolved is to make sure all of the USB-related 
devices (including the root hub) have a device_detach method implemented.

uhub is easy enough to fix -- like OHCI and EHCI it has a detach function 
written but it's just not referenced in the device_method_t.

The usb device is a whole can of worms though.  It has a detach method that 
just spits out "unload prevented" and returns EINVAL.  Of course since the 
usb bus has physically been removed it doesn't have much of a choice :).  I 
put in a null function that just returns 0 as a test, but it either panics 
immediately because the event thread is still polling the device or panics 
when you put the card back in and it tries to make_dev something that already 
exists ("panic: don't do that").

It looks like the NetBSD and OpenBSD cases do have detach code, so it may just 
be a matter of modifying that remove the device entry.  If I get some free 
time tomorrow I'll take a crack at it and see how many times I can panic this 
poor laptop :D

Craig