ALSA: usb-audio: Unify the release of usb_mixer_elem_info objects

Instead of the direct kfree() calls, introduce a new local helper to
release the usb_mixer_elem_info object.  This will be extended to do
more than a single kfree() in the later patches.

Also, use the standard goto instead of multiple calls in
parse_audio_selector_unit() error paths.

Signed-off-by: Takashi Iwai <tiwai@suse.de>