ALSA: usb-audio: remove invalid extra mixers for Komplete Audio 6

This was a flaw in the reading of the spec tables - Native Instrument's
"Komplete Audio 6" device has no such extra controls.

This patch also fixes the device name in two comments.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>