ALSA: firewire-tascam: send fixed-length transaction for async midi port

TASCAM FireWire series uses asynchronous transactions with fixed length
payload for MIDI messaging. On the other hand, ALSA driver for the series
has a redundant design to handle different length of payload.

This commit removes the redundant abstraction.

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