ALSA: fireface: code refactoring to add enumeration constants for model identification

In RME fireface series, version field of unit directory in configuration
ROM is used to distinguish each model. The value of field is known and
it's better to use enumeration constants for code representation.

This commit adds enumeration constants for model identification.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20200510074301.116224-4-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Iwai <tiwai@suse.de>