ALSA: firewire-tascam: remove needless member for control and status message

Commit 3beab0f8 added a member for control and status message, while
it's planned and not implemented yet.

This commit removes it.

Fixes: 3beab0f8

('ALSA: firewire-tascam: add support for outgoing MIDI messages by asynchronous transaction')
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>