[media] rtl28xxu: enable demod ADC only when needed

Enable integrated demod ADC only when demod is used. Keep integrated
demod ADC disabled when external demod is used. This fixes corrupted
stream in a case external demod was used.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>