ols: Fix demuxer operation

ols: Fix demuxer operation

On my OLS with Demon Core (v3.07), the following command produces only low
levels on channels 8-15:
sigrok-cli -d ols:conn=/dev/ttyACM0 --output-format ascii:width=128 \
  --config='samplerate=200M:pattern=Internal:captureratio=50' \
  --channels 0-15 --samples 32 --triggers "12=1"

This doesn't make sense since it only triggers if channel 12 is high.
By not overwriting the channel data, I get the desired output. The data
is processed in groups of 2 bytes, so there's no need to fold back "input"
from devc->sample[2..3] because these bytes are always zero.