bluetooth: fix set_volume_cb on sco over pcm
The current implementation is totally bogus, it cast the over_sink
userdata to the bluetooth-device userdata... It was failing nicely
because the previous code had a gentle safe-guard in u->profile ==
PROFILE_HSP, and u->profile was just random.
There is no easy way to associate additional data to a sink or
source. Two solutions seems possible: looking up loaded modules and
check which one was handling the sink/source, or using pa_shared. I
went for the second solution.