hdspmixer: Fix preset activation segfault on AES(32)
When selecting preset 6 or 7 on AES(32), hdspmixer has caused a segfault
due to indirect out of bound access on the destination label array.
The amount of destinations is the number of physical stereo
pairs, so it's usually half the channel count, in some cases one more if
there are additional headphone jacks.
Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de> Signed-off-by: Takashi Iwai <tiwai@suse.de>