ALSA: usb-audio: make hwc_debug a noop in case HW_CONST_DEBUG is not set

Just defining it to nothing is dangerous as it can alter the code
execution flow, for example when used in as only function in a
conditional code block.
Signed-off-by: NDaniel Mack <zonque@gmail.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/usb/debug.h

@@ -8,7 +8,7 @@
 #ifdef HW_CONST_DEBUG
 #define hwc_debug(fmt, args...) printk(KERN_DEBUG fmt, ##args)
 #else
-#define hwc_debug(fmt, args...) /**/
+#define hwc_debug(fmt, args...) do { } while(0)
 #endif
 
 #endif /* __USBAUDIO_DEBUG_H */