Bluetooth: bpa10x: Free private driver data on usb shutdown

Instead of waiting for the hci-device to be destroyed we now free the
private driver data on driver shutdown right away. We call
hci_unregister_dev() on driver shutdown, that means, the hci-core will
never ever call our callbacks again except the destruct callback. It
also does not access hdev->driver_data so there is no reason to keep
that alive. We simply set the destruct cb to NULL to avoid getting
called again.

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>