V4L/DVB (12874): pvrusb2: disable tda18271 slave tuner output / loop thru in standby mode

Enable the standby mode optimization to disable the tda18271
slave tuner output / loop thru options when in low power mode

Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>