[media] rtl28xxu: use master I2C adapter for slave demods

Both mn88472 and mn88473 slave demods are connected to master I2C
bus, not the bus behind master demod I2C gate like tuners. Use
correct bus.

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