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: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>