When traversing the color balance element channel list to find the one that
matches with the playsink proxy, the assignation was set to iterator of the
playsink proxy, not the balance element. Thus, the mapping to the values of
the balance element channel was wrong.
This patch fixes the assignation of the color balance element channel, so the
mapping to the channel of the color balance element is fixed.
https://bugzilla.gnome.org/show_bug.cgi?id=750691