Bluetooth: Introduce a flag to track who really initiates authentication

Even though our side requests authentication, the original action that
caused it may be remotely triggered, such as an incoming L2CAP or RFCOMM
connect request. To track this information introduce a new hci_conn flag
called HCI_CONN_AUTH_INITIATOR.

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