Fix another off-by-one error.
Fix another off-by-one error.

At the C library level, channel/dac is always zero indexed.

Also updating APS_SetChannelOffset to use a normalized offset parameter
(-1,1).

--BRJ and CAR

BBN-Q/libaps2,BBN-Q/libaps2,BBN-Q/libaps2,BBN-Q/libaps2,BBN-Q/libaps2