ALSA: firewire-tascam: remove a flag for controller

Currently, 'struct snd_tscm_spec' has a member named as 'is_controller' to
identify MIDI controller. This member was originally added to skip
parse control and status messages in isochronous packets for non-controller
model.

As long as I investigate, FW-1804 (non-controller) also transfers the
control and status message, thus it becomes meaningless.

This commit removes it.

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