ALSA: firewire-digi00x: add MIDI operations for MIDI control port

Digi 002/003 family has two types of MIDI port; one is for physical MIDI
port and another is for MIDI control message. The former is transferred in
isochronous packet, and the latter is transferred by asynchronous
transaction. These transmission mechanisms are completely different, while
current ALSA digi00x driver defines a set of operations for them with
several condition statements. As a result, codes for the operation are
messy.

This commit adds a set of MIDI operation for control MIDI ports. In later
commit, it's applied as an operation for ALSA rawmidi character device.

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