Bluetooth: Controller: Implement bt_hci_driver::close

The open source controller did not expose support for shutting it down
via the close() API in the HCI driver. Add support for it, which
completes the support for disabling and enabling the Bluetooth stack in
Zephyr.

Closes #3192.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>