[media] cx231xx: prepare for i2c_client attachment

This is needed to support PCTV QuatroStick 522e which uses a si2157.
The si2157 driver is written using i2c_client attachment.

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Tested-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>