Bluetooth: Add missing cmd_status handler for LE_Start_Encryption

It is possible that the HCI_LE_Start_Encryption command fails in an
early stage and triggers a command status event with the failure code.
In such a case we need to properly notify the hci_conn object and
cleanly bring the connection down. This patch adds the missing command
status handler for this HCI command.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>