Bluetooth: mgmt: Make Set Link Security callable while powered off

This patch makes it possible to change the Link Security setting while
powered off and have it automatically enabled when powering on a device.
To track the desired state once powered on a new HCI_LINK_SECURITY flag
is added.

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