ALSA: usb-audio: remove some dead code

We recently cleaned up the error handling in commit 52c3e317 ("ALSA:
usb-audio: Unify the release of usb_mixer_elem_info objects") but
accidentally left this stray return.

Fixes: 52c3e317 ("ALSA: usb-audio: Unify the release of usb_mixer_elem_info objects")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>