This is a revised version of an earlier patch; I feel a lot better about
this one.  Basically it does the same thing as before: allocate
interfaces dynamically to avoid the problems with reusing them.

The difference is that this patch adds a struct kref to the array of
usb_interface_cache's, so the array can persist if needed after the
device has been disconnected.  Each interface takes a reference to it
(along with the configuration itself), so as long as the interfaces
remain pinned in memory the altsettings will also remain.

Here is a slight revision of patch as246b.  This one allocates all the new
interfaces before changing any other state; otherwise it's the same.