ALSA: usb-audio: Replace hard-coded number with MAX_CHANNELS

ALSA: usb-audio: Replace hard-coded number with MAX_CHANNELS

One place in mixer.c still used a hard-coded number 16 instead of
MAX_CHANNELS.  Replace with it, so that we can extend the max number
of channels gracefully.

Link: https://lore.kernel.org/F1B104A5-CD6A-4A26-AB46-14BF233C0579@getmailspring.com
Tested-by: Phil Willoughby <willerz@gmail.com>
Link: https://patch.msgid.link/20260327153056.691575-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>