ALSA: usb-audio: Fix imbalance per-channel volume of sticky mixers

ALSA: usb-audio: Fix imbalance per-channel volume of sticky mixers

I accidentally made an off-by-a-line mistake when mimicking other code
paths that set all channels. The mistake breaks sticky mixers with
multiple channels.

I didn't realize this mistake at that time, as my device's mixer is
single-channel.

Fix it, so that per-channel volume of sticky mixers is balanced.

Fixes: aa2f4addab44 ("ALSA: usb-audio: Set the value of potential sticky mixers to maximum")
Signed-off-by: Rong Zhang <i@rong.moe>
Link: https://patch.msgid.link/20260706-uac-sticky-channels-fix-v1-1-92741c538283@rong.moe
Signed-off-by: Takashi Iwai <tiwai@suse.de>