hameg-hmo: Do read from second digital pod during acquisition

hameg-hmo: Do read from second digital pod during acquisition

The previous implementation only added one of the digital channels to
the list of enabled channels that are involved in the acquisition (the
first one that was found). This means that when the set of used digital
channels spans more than one pod/group, the second pod will never be
read from.

Make sure to enable one digital channel per pod/group, such that
acquisition will retrieve data from all involved input sources.

Add comments while we are here. Mention how the different setup, check,
start, and receive routines which are spread across several files do
interact to achieve acquisition.