Bluetooth: Simplify Link Key Notification event handling logic

When we get a Link Key Notification HCI event we should already have a
hci_conn object. This should have been created either in the Connection
Request event handler, the hci_connect_acl() function or the
hci_cs_create_conn() function (if the request was not sent by the
kernel).

Since the only case that we'd end up not having a hci_conn in the Link
Key Notification event handler would be essentially broken hardware it's
safe to simply bail out from the function if this happens.

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