ALSA: firewire-tascam: remove callback function from async midi port

As a result of localization of async midi port, ALSA driver for TASCAM
FireWire series can call helper function directly instead of callback
registration.

This commit removes the redundant design.

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