V4L/DVB (8649): v4l2: add AAC bitrate control

If you can select AAC as audio encoder, then you should also be
able to set the bitrate. Add this missing control.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>