[media] si2168: carry pointer to client instead of state

Carry struct i2c_client pointer inside struct dvb_frontend private
pointer. This driver is I2C driver, which is represented as a
struct i2c_client, so better to carry this top level structure for
each routine in order to unify things. This allows further
simplification.

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