[media] rtl2832_sdr: control ADC

Recent rtl28xxu patch I made moved demod ADC enable from power control
to frontend control (due to slave demod support). Because of that we
need call USB interface frontend control too in order to enable ADC.

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