l2tp: Fix set-but-unused variables.

The variable 'ret' is set but unused in l2tp_nl_register_ops().

This was obviously meant to maintain error codes which are
returned to the caller, make it so.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_netlink.c

@@ -795,11 +795,12 @@ int l2tp_nl_register_ops(enum l2tp_pwtype pw_type, const struct l2tp_nl_cmd_ops
 		goto out;
 
 	l2tp_nl_cmd_ops[pw_type] = ops;
+	ret = 0;
 
 out:
 	genl_unlock();
 err:
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL_GPL(l2tp_nl_register_ops);