[media] rtl28xxu: simplify FE callback handling

Logic is so simple that there is no idea to separate tuner selection to
own function, instead do it in a callback and get rid of one function.

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