output/csv: use longer names for iteration variables

output/csv: use longer names for iteration variables

The process_analog() logic is rather complex, dealing with the total
list of channels in the device (which can be of different types), and a
number of submitted samples for a specified list of channels. Replace
the rather short variable names for i, j, c (and num_channels) with
something longer that hopefully increases readability of the complex
loop bodies.

Note that this change merely renames identifiers, and does not change
behaviour.