decoder: accept skip count 0 (onewire_link, usb_signalling)

decoder: accept skip count 0 (onewire_link, usb_signalling)

A recent commit tightened the check for acceptable 'skip' sample counts.
This broke the onewire_link and usb_signalling decoders' test sequences,
which no longer pass with a minimum count of one. Relax the condition
and accept a count of zero. This breaks gpib again for low total sample
count option values, but this needs to get addressed differently.