The error from snd_usb_mixer_apply_create_quirk() is ignored in the
current usb-audio driver code, which will continue the probing even
after the error.  Let's take it more serious.

Fixes: 7b1eda22 ("ALSA: usb-mixer: factor out quirks")
Signed-off-by: Takashi Iwai <tiwai@suse.de>