ALSA: mixart: don't print an unintialized variable on error

My static checker complains that "resp" could be unitialized on error
when we print its value.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>