V4L/DVB (4758): Cx88: use external adc for svideo/composite

For the KWorld HardwareMpegTV XPert, the external adc must be used for
svideo / composite inputs, but television / radio inputs use the internal adc.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>