Midi: only output PORT if midiChannelMapping == #'voice.

Midi: only output PORT if midiChannelMapping == #'voice.

This reduces the risk of creating trouble for the default
#'instrument-mode output and the old #'staff-mode output,
while leaving the option open for MIDI-players to do the
right thing with ports in the #'voice-mode: in each track,
each channel 0 should be able to play another instrument.