ALSA: dice: add MIDI ports according to current number of MIDI substreams

This commit changes the way to add ALSA MIDI ports. This driver read the
number of multiplexed MIDI substreams from hardware register, then adds the
same number of ALSA MIDI ports. This commit is based on my assumption that
the number is fixed at all of supported sampling transfer frequency.

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