Bluetooth: Convert delayed discov_off to hci_sync

The timed ending of Discoverability was handled in hci_requst.c, with
calls using the deprecated hci_req_add() mechanism. Converted to live
inside mgmt.c using the same delayed work queue, but with hci_sync
version of hci_update_discoverable().

Signed-off-by: Brian Gix <brian.gix@intel.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>