ALSA: dice: use stream formats to add MIDI substreams

In former commits, proxy structure gets members for cache of stream
formats. The cache can be used to count the number of MIDI substreams
to add.

This commit uses the cache for this purpose.

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