WUSB: disconnect all devices when stopping a WUSB HCD

Make sure all WUSB devices are disconnected when stopping a WUSB HCD so
that we don't leak the devices' wusb_dev structures.

Signed-off-by: David Vrabel <david.vrabel@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>