In Mp, can't get correct Struck SIS3820 is_busy() value from a thread

[jbhopkins]

As title. Calling struck.is_busy() from a thread using Mp doesn't return the same value as it does when using EPICS caget from the command line. In particular, it returns something like 32578, and so defaults to always busy, regardless of the actual status (which should be 0 or 1).