ALSA: dice: get the number of MBLA data channel at opening PCM substream

This commit is a preparation to remove members related to channel cache
for the number of channels for multi bit linear audio data and MIDI
ports. This commit changes the way to get the number of multi bit linear
audio data channel. It's directly retrieved by asynchronous transactions
to some registers.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>