ALSA: usb-audio: Add private_data pointer to usb_mixer_elem_info

Add a private_data pointer to usb_mixer_elem_info to allow other mixer
implementations to extend the structure as necessary.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>