dca: factorize scaling in inverse ADPCM
Based on a patch from Christophe Gisquet. Unrolling of the m == 0 case avoids a possible use of the uninitilized value sum when s->predictor_history is not set. I failed to find a sample for it. It also reduced the cycle count from 220 to 150 on sandy bridge, x86_64 linux, gcc 4.8.2 compared to his patch.
Showing with 19 additions and 8 deletions