ALSA: usb: Avoid VLA in mixer_us16x08.c

This is another attempt to work around the VLA used in
mixer_us16x08.c.  Basically the temporary array is used individually
for two cases, and we can declare locally in each block, instead of
hackish max() usage.

Signed-off-by: Takashi Iwai <tiwai@suse.de>