[media] tda18212: rename state from 'priv' to 'dev'

foo_dev seems to be most correct term for the structure holding data
of each device instance. It is most used term in Kernel codebase and also
examples from book Linux Device Drivers, Third Edition, uses it.

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