Fix vchan speed for hardware with discrete (non-continuous) sampling rate: - Improve vchan chn_setspeed() strategy. Try to avoid FEEDER_RATE   on parent channel if the requested value is not supported   by the hardware. - Fix vchan default speed calculation. In any case, vchan should   rely on parent bufsoft speed instead of bufhard since it is   possible that the entire feeder chain might involve FEEDER_RATE.   This is possible under extreme, rare condition if the above   chn_setspeed() strategy failed.
Fix vchan speed for hardware with discrete (non-continuous)
sampling rate:
- Improve vchan chn_setspeed() strategy. Try to avoid FEEDER_RATE
  on parent channel if the requested value is not supported
  by the hardware.
- Fix vchan default speed calculation. In any case, vchan should
  rely on parent bufsoft speed instead of bufhard since it is
  possible that the entire feeder chain might involve FEEDER_RATE.
  This is possible under extreme, rare condition if the above
  chn_setspeed() strategy failed.

Approved by:	netchild (mentor)

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase