Bluetooth: Add hci_conn flag for new link key generation

For LE Secure Connections we want to trigger cross transport key
generation only if a new link key was actually created during the BR/EDR
connection. This patch adds a new flag to track this information.

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