V4L/DVB (12487): Fix control AC-3 of the 6752HS

Fix error for AC-3 control of the 6752HS MPEG-2 coder.

Signed-off-by: Beholder Intl. Ltd. Dmitry Belimov <d.belimov@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>