tcm_unregister_callback
if (opts->tcm_unregister_callback)
opts->tcm_unregister_callback(f);
opts->tcm_unregister_callback = tcm_unregister_callback;
void (*tcm_unregister_callback)(struct usb_function_instance *);
tcm_opts->tcm_unregister_callback = usbg_detach;