[media] rtl28xxu: rename tuner I2C client pointer

Better to rename tuner I2C to something which clearly says it is
for tuner as there is now multiple different I2C clients used.

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