[media] rtl28xxu: fix DVB FE callback

DVB FE callback functionality went broken after I moved tuners to
demod muxed I2C adapter. That happens because driver state was
carried by I2C adapter and when mux is used there is one adapter
more in a chain.
USB adapter <-> I2C adapter <-> muxed I2C adapter

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