The actual problem reported was because there wasn't a corresponding
check on transport_classdev.class in the unregister.

However, on closer inspection I also turned up a nasty thinko in the
reference counting.  For reasons best known to the class code authors,
class devices have to obtain their own references to the devices they're
attached to which they release again in their .release routines, so you
have to remember to do a get_device() in the correct place after the
class_device_add().  I put comments in the code so that, hopefully, we
can avoid the problem in future.