ALSA: usb-audio: Extend max number of channels to 64

ALSA: usb-audio: Extend max number of channels to 64

The current limitation of 16 as MAX_CHANNELS is rather historical at
the time of UAC1 definition.  As there seem already devices with a
higher number of mixer channels, we should extend it too.  As an ad
hoc update, let's raise it to 64 so that it can still fit in a single
long-long integer.

Link: https://lore.kernel.org/F1B104A5-CD6A-4A26-AB46-14BF233C0579@getmailspring.com
Tested-by: Phil Willoughby <willerz@gmail.com>
Link: https://patch.msgid.link/20260327153056.691575-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>