[media] tda18212: prepare for I2C client conversion

We need carry pointer to frontend via config struct
(I2C platform_data ptr) when I2C model is used. Add that pointer
first in order to keep build unbreakable during conversion.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>