mixer: fix a couple of pessimizations bugs in the 8-channel mixer
mixer: fix a couple of pessimizations bugs in the 8-channel mixer

1) The 8-channel mixer core was not called for exactly 8
channels but only up to 7.

2) The last iteration of the 8-channel mixer core would run as a
32-channel mixer.

sp1187/libdragon,DragonMinded/libdragon,DragonMinded/libdragon,DragonMinded/libdragon,sp1187/libdragon,sp1187/libdragon