[media] tda18271: enter low-power standby mode at the end of tda18271_attach()

Ensure that unnecessary features are powered down at the end of the
attach() function.  If the configuration requires the loop thru or
xtout features, they will remain enabled.
Thanks to Antti Palosaari for noticing the additional power consumption.

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