Made it possible to request a mite channel from a specific range,

Made it possible to request a mite channel from a specific range,
instead of just starting at a specified offset.  Only allow
use of mite channels 0-3 for ai/ao, since those are the only
ones that seem to work.  Commented out 3 ai convert pulses sent
when clearing ai fifo on 625x boards (based on NI example code),
since it appears to be wrong when tested with real hardware.