Bluetooth: Move arming of discoverable timeout to complete handler

The discoverable timeout is currently armed from hci_event.c and causes
some side effects when using HCI commands instead of the management
interface. To make this clear, only arm the discoverable timeout from
the management command complete handler.

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