Modify US digital frequency tables to allow channels up to 159

[sphery <mtdean@…>]

The attached patch modifies the US digital frequency tables (standard, HRC, and IRC) to include channels up to 159. Currently, the HRC and IRC tables stop at channel 125.

In making the change, I defined US_MAX_CHAN and EIA_542_FREQUENCY (an equation to compute the appropriate center frequency based on the EIA-542 standard's definition--it requires a bandwidth, offset, and channel number). This equation (with appropriate bandwidth and offset for standard, HRC, or IRC) is valid for any channel over 99. Since US_MAX_CHAN is never likely to be reduced (and reduction to below 100 doesn't make sense on a cable frequency table), this shouldn't be an issue.

After this change, if we ever need to allow additional channels in the future, we simply need to modify US_MAX_CHAN. In doing so, we'll update all three standards' frequency tables.

I didn't modify channel 100's frequency (the band's starting channel) to use the EIA_542_FREQUENCY define, although doing so work (and would be easy).

[danielk]

(In [11728]) Fixes #2624. Extends IRC and HRC frequency tables from a max of 125 to 159 using a macro which allows increasing this further easily, using patch from mtdean.