V4L/DVB (10876): tda18271: add support for AGC configuration via tuner callback

The tda827x driver supports a feature that the tda18271 driver was lacking
until now.  This patch adds support for device-level configuration via the
tuner callback configuration interface.

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