hameg-hmo: Fix index access for models with 2 pods / 16 digital channels

hameg-hmo: Fix index access for models with 2 pods / 16 digital channels

Fix the code which registers the name of the second pod for digital
probes. The previous implementation registered the first pod twice, and
lost the reference to the second pod. No harm was done, none of the
supported models declared support for two pods so far.

Factor out a channel to group mapping in the registration of digital
channels, while we are here.