Bluetooth: hci_sync: Convert MGMT_OP_SET_DISCOVERABLE to use cmd_sync

This makes MGMT_OP_SET_DISCOVERABLE use hci_cmd_sync_queue instead of
use a dedicated discoverable_update work.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>