ALSA: usb-audio: localize function without external linkage

When accessed inner a file, functions should have static qualifier for
local-linkage.

This commit fixes the bug. Sparse generated below warning.

sound/usb/mixer_us16x08.c:1043:32: warning: symbol 'snd_us16x08_create_meter_store' was not declared. Should it be static?

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>