usbfs: detect device unregistration

This patch (as711b) is a revised version of an earlier submission.  It
modifies the usbfs code to detect when a device has been unregistered from
usbfs, even if the device is still connected.  Although this can't happen
now, it will be able to happen after the upcoming changes to usb_generic.

Nobody objected to this patch when it was submitted before, so it should
be okay to apply this version.  The revision is merely to take into
account the changes introduced by as723, which touches the same driver.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>