Bluetooth: Add SC-only mode support for SMP

When Secure Connections-only mode is enabled we should reject any
pairing command that does not have Secure Connections set in the
authentication requirements. This patch adds the appropriate logic for
this to the command handlers of Pairing Request/Response and Security
Request.

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