Some crappy USB-audio devices give broken dB ranges, e.g. both min and max
are 0dB.  This confuses the volume control that prefers dB expression such
as alsactl or PulseAudio.  In such a case, it's much better not to expose
the broken dB information.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>