sound: Remove unnecessary casts of private_data

sound/core/oss/mixer_oss.c

sound/core/pcm.c

sound/drivers/virmidi.c

sound/i2c/other/ak4xxx-adda.c

sound/isa/ad1816a/ad1816a.c

sound/isa/azt2320.c

sound/isa/gus/gusmax.c

sound/isa/sb/sb8.c

sound/oss/au1550_ac97.c

sound/pci/emu10k1/emumpu401.c

sound/pci/ice1712/pontis.c

sound/pci/ice1712/prodigy192.c

sound/pci/rme96.c

sound/pci/rme9652/hdsp.c