ALSA: Au88x0 - Reduce the number of playback subdevices of au8830 from 32 to 16

- The maximum number of playback streams depend on the number of sample
  rate conveters (16) and the number of DMA channels (32).

Signed-off-by: Raymond Yau <superquad.vortex2@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>