In some rare circumstances it was possible for the host software to
become so busy that it does not transmit a get_clock request for
several seconds. (In particular, this could occur with some complex
calls to coordinate_descent.) If that happened, it was possible for
the code to incorrectly report a "Timeout with MCU" error. Rework the
is_active() check to prevent that.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>