As the recent fix addressed the channel swap problem more properly,
update the comment as well.

Fixes: 1b7ecc24 ("ALSA: usb-audio: work around streaming quirk for MacroSilicon MS2109")
Signed-off-by: Hector Martin <marcan@marcan.st>
Link: https://lore.kernel.org/r/20200816084431.102151-1-marcan@marcan.stSigned-off-by: Takashi Iwai <tiwai@suse.de>