mixer-control: Fix extra reference being leaked

In both cases objects are inserted in hash table by adding an
extra ref, but the existing reference was never removed. Don't
add a reference instead, so we don't end up with a spare one.

Based on patch by Alberts Muktupāvels <alberts.muktupavels@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=765694