[media] cx18: Only allocate a struct cx18_dvb for the DVB TS stream

The cx18_stream struct contained a struct cx18_dvb for
every stream object, most of which were for analog capture.
Now we only allocate the cx18_dvb object for the DTV TS stream.

Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>