USB: cxacru: document how to interact with the flash memory

These commands were found by accident... fortunately
it still works even if the flash memory is erased,
despite having no USB device IDs.

Some example sysfs code for raw command access:
  http://simon.arlott.org/pub/cxacru/raw.c



Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>