demo: Cast to double while calculating the pattern frequency.

Without the cast non integer frequencies weren't possible (e.g. with a sampling
frequency of 50Hz we would end up with a signal frequency of 2Hz instead of
2.5Hz). The result were signals which had an incorrect number of samples per
period.

BugLink: http://sigrok.org/bugzilla/show_bug.cgi?id=297
scpi: Limit the log message in get_string() to 70 characters.