Add index temporary variable to scarlett2_mixer_ctl_put() for
consistency with the other *_ctl_put() functions.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/20210620164641.GA9211@m.b4.vu


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