V4L/DVB (13201): saa7164: disable tda18271 slave tuner output on slave in standby mode

Enable the standby mode optimization to disable the tda18271
slave tuner output / loop thru options when in low power mode
on the slave tuner, only.  The master tuner must always leave
slave tuner output / loop thru enabled.

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