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) since these do pass pointers to struct
raw1394_reqhandle as the tag and expect the callback to be invoked.