[media] rtl2832: implement sleep

Put demod to soft reset in order to save power when sleep. That drops
power usage ~30mA @5V on USB dongle I tested. In real life it does
not matter much as USB IF powers off demod too, but now it is done
twice - demod and USB IF.

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