Bluetooth: btusb: Free driver data on USB shutdown

Instead of using the hci-destruct callback we free our private driver
data on USB shutdown. We already called hci_unregister_dev() here so the
hci core will never ever call our callbacks again except the destruct
callback.

However, there is no reason to keep our *private* driver data alive if
we get never called again and the hci-core does never touch it the data.
So we simply free it right away and set the destruct callback to NULL.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>