[media] tda8290: Allow disabling I2C gate

Allow disabling I2C gate handling by external configuration.
This is required by cards that have all devices on a single I2C bus,
like AverMedia A706.

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