ALSA: fireface: constify ALSA specific operations

ALSA fireface driver has ALSA specific operations for MIDI/PCM data.
Structured data for the operations can be constified. Additionally,
The structured data can be function local.

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