ALSA: oxfw: code refactoring to detect mackie models

This commit changes condition statement to call mackie models detection
just for the device entry. Additionally, comment is added for Onyx 1640i.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210518084557.102681-5-o-takashi@sakamocchi.jp


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