Change setchannels 1 behavior

[dihm]

Currently, firmware will mirror instructions to all channels if setchannels 1 has been used. This means it is impossible to only have output on a single channel. I think a better solution is to ensure setchannels 1 only allows output on 1 channels, and to preserve this functionality by allowing setchannels 0 which indicates instructions should be mirrored to all outputs.