[media] au0828: remove control buffer from send_control_msg

There are no cases where a control message is ever sent to the au0828
with an actual buffer defined.  Remove the reference to dev->ctrlmsg,
which currently requires us to hold a mutex since it is shared with the
read function.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>