Fix the cable-card mode for the 6-tuner Ceton.

The new Ceton cards don't like being told to tune to frequency 0 when using
qam_256.  This problem was diagnosed/fixed by matt @ mossholder.com, and
reviewed/cleaned up by Ronald Frazier.  Thanks guys!

Fixes #11815

(cherry picked from commit 43ccf35a9c70310952259afb732ab86669805c39)