[iscsi] Use intfs_shutdown() when shutting down multiple interfaces

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp/iscsi.c

 	process_del ( &iscsi->process );
 
 	/* Shut down interfaces */
-	intf_nullify ( &iscsi->data ); /* avoid potential loops */
-	intf_shutdown ( &iscsi->socket, rc );
-	intf_shutdown ( &iscsi->control, rc );
-	intf_shutdown ( &iscsi->data, rc );
+	intfs_shutdown ( rc, &iscsi->socket, &iscsi->control, &iscsi->data,
+			 NULL );
 }
 
 /**