ALSA: usb-audio: localize one-referrer variable

When accessed by one referrer inner a file, variables should have static
qualifier to declare local-linkage.

This commit fixes the bug. Sparse generated below warnings.
sound/usb/mixer_us16x08.c:156:13: warning: duplicate const
sound/usb/mixer_us16x08.c:156:18: warning: symbol 'route_names' 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>