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: d2bb390a20 ("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>

sound/usb/mixer_us16x08.c

@@ -153,7 +153,7 @@ static const char ratio_map[] = {
 };
 
 /* route enumeration names */
-const const char *route_names[] = {
+static const char *const route_names[] = {
 	"Master Left", "Master Right", "Output 1", "Output 2", "Output 3",
 	"Output 4", "Output 5", "Output 6", "Output 7", "Output 8",
 };