Bluetooth: bfusb: Free driver_data on USB shutdown

This frees the private driver data on USB shutdown instead of using the
hci-destruct callback. We already call usb_set_intfdata(intf, NULL) but
we do not do the same with the hci object. This would be totally safe,
though.

After calling hci_unregister_dev()/hci_free_dev() the hdev object will
never call any callback of us again except the destruct callback.
Therefore, we can safely set the destruct callback to NULL and free the
driver data right away.  This allows to unload the module without
waiting for the hdev device to be released.

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>