[media] siano: Only feed DVB data when there's a feed

Right now, the driver sends DVB data even before tunning.
It was noticed that this may lead into some mistakes at DVB
decode, as the PIDs from wrong channels may be associated with
another frequency, as they may already be inside the PID buffers.
So, prevent it by not feeding DVB demux with data while there's no
feed or while the device is not tuned.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>