Bluetooth: Add mgmt_set_secure_conn support for any LE adapter

Since LE Secure Connections is a purely host-side feature we should
offer the Secure Connections mgmt setting for any adapter with LE
support. This patch updates the supported settings value and the
set_secure_conn command handler accordingly.

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