ALSA: firewire-digi00x: rename identifiers of MIDI operation for physical ports

In following commit, new functions and variables are added for operations
of MIDI control port.

This commit is a preparation. Current identifiers are renamed so that they
mean physical MIDI ports.

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