[media] tda18271: make 'low-power standby mode after attach' multi-instance safe

Ensure that unnecessary features are powered down at the end of the
attach() function on attach of the tuner's first instance. If the
configuration requires the loop thru or xtout features, they will
remain enabled.
This must *only* be done after attaching the first instance of the tuner.
If there are multiple instances of the tuner, the bridge driver will need
to maintain power managament by itself.

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