demo: Skip generating data when all channels in a group are disabled

demo: Skip generating data when all channels in a group are disabled

The generator logic determines how many samples per group (analog and
logic) need to get produced, then keeps iterating until each group has
reached the specified count. Different groups can generate different
numbers of samples per iteration, they have their own stride.

It's essential to check whether all channels in a group are disabled, to
then completely skip the respective half of the generation loop. Without
this check, the group would be expected to generate data but it won't,
which results in an endless loop. This was observed with analog channels.

There was another issue with logic channels. Unexpected logic data was
seen in the output although neither logic channel was selected.

This commit fixes bug #923.