ALSA: firewire-tascam: move message parameters for async midi port

Units on TASCAM FireWire series handle MIDI messages with support for
running status. Drivers for the series should remember current running
status and transfer valid MIDI messages. For this purpose, current
ALSA driver for the series has some members in its top-level structure.
This is due to better abstraction of async midi port. Nowadays, the
abstraction was localized just for the driver.

This commit moves the members to structure for async midi port.

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