Bluetooth: uart-ldisc: Fix memory leak and remove destruct cb

We currently leak the hci_uart object if HCI_UART_PROTO_SET is never set
because the hci-destruct callback will then never be called.  This fix
removes the hci-destruct callback and frees the driver internal private
hci_uart object directly on tty-close. We call hci_unregister_dev() here
so the hci-core will never call our callbacks again (except destruct).
Therefore, we can safely free the driver internal data 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>