ALSA: firewire-digi00x: allow user space applications to read/write MIDI messages for all ports

At a commit c5fcee03

 ("ALSA: firewire-digi00x: add MIDI operations for
MIDI control port"), I described that MIDI messages for control surface is
transferred by a different way from the messages for physical ports.
However, this is wrong. MIDI messages to/from all of MIDI ports are
transferred by isochronous packets.

This commit removes codes to transfer MIDI messages via asynchronous
transaction, from MIDI handling layer.

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