ALSA: firewire-digi00x: add another rawmidi character device for MIDI control ports

Digi 002/003 family uses two ways to transfer MIDI messages. They're
different mechanisms, while it's better to handle the ways in different
ALSA rawmidi character devices because one character device has just a
set of operations.

This commit adds another rawmidi character device for control MIDI port. As
a result, first rawmidi character device is just for MIDI messages
transferred by isochronous packets.

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