ALSA: fireface: add protocol-specific operation to fill transaction buffer with MIDI messages

Between former and latter models, content of asynchronous transaction
for MIDI messages from driver to device is different.

This commit is a preparation to support latter models. A protocol-specific
operation is added to encode MIDI messages to the transaction.

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