[media] si2157: carry pointer to client instead of state in tuner_priv

Carry struct i2c_client pointer in tuner_priv. 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.

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