ALSA: Remove invalid GENERIC_MIX PCM sublass

SNDRV_PCM_SUBCLASS_GENERIC_MIX is mostly for h/w multi-stream playback
devices, but ca0106 and emu10k1x don't support it (unlike emu10k1).
We shouldn't set that flag to avoid confusion.

Signed-off-by: Takashi Iwai <tiwai@suse.de>