The name 'probe' (and thus 'probe group') is a relic from the times when
sigrok was mostly about logic analyzers. Nowadays we support a lot more
device types where 'probe' is not really a good term and 'channel' is
much better suited.
This fixes parts of bug #259.
Can't really count on it either way though, the device is just
too flaky to conclude whether it's going to work or not, regardless
of the status returned.