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)
6ef4b2f555