V4L/DVB (11805): au0828: send command to power down tuner when done with analog

Make sure the au0828 issues the command to power down the tuner when the
user is done using analog support.

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