ALSA: fireface: add support for MIDI functionality

In previous commit, fireface driver supports unique transaction mechanism
for MIDI feature. This commit adds MIDI functionality for userspace
applications.

As I wrote in a followed commit, user space applications get some
requirement from this driver. It should not touch a register to which
units transmit MIDI messages. It should configure a register in which
MIDI transmission is controlled.

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