ALSA: usb-audio - Fix an unused-variable compile warning

Used only when CONFIG_SND_DEBUG=y
  sound/usb/mixer.c: In function 'get_min_max':
  sound/usb/mixer.c:762: warning: unused variable 'chip'

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>