[iscsi] Avoid potential infinite loops during shutdown

The command and data interfaces may be connected to the same object.
Nullify the data interface before shutting down the control interface
to avoid potential infinite loops.

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 );