Sets the handler to be called whenever a request completes to new_h. The default handler interprets the tag as a pointer to a &struct raw1394_reqhandle and calls the callback in there.
Care must be taken when replacing the tag handler and calling the synchronous versions of the transaction functions (i.e. raw1394_read, raw1394_write, raw1394_lock, raw1394_iso_write) since these do pass pointers to &struct raw1394_reqhandle as the tag and expect the callback to be invoked.