ALSA: usb-audio: purge needless variable length array

Variable length array is used in 'snd_us16x08_meter_get()', while there
is no need. It's better to purge it because variable length array has
overhead for stack handling.

This commit replaces the array with static length. Sparse generated below
warning.

sound/usb/mixer_us16x08.c:714:18: warning: Variable length array is used.

Fixes: d2bb390a

 ("ALSA: usb-audio: Tascam US-16x08 DSP mixer quirk")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>