ALSA: usb-audio: make set_*_mix_values functions public

Make the functions set_cur_mix_value and get_cur_mix_value accessible by files
that include mixer.h. In addition make usb_mixer_elem_free accessible.
This allows reuse of these functions by mixers that may require quirks.

The following summarizes the renamed functions:
  - set_cur_mix_value -> snd_usb_set_cur_mix_value
  - get_cur_mix_value -> snd_usb_get_cur_mix_value
  - usb_mixer_elem_free -> snd_usb_mixer_elem_free

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