[media] lgs8gxx: don't export get_frontend() callback

This device doesn't really have a get_frontend(). All it does is
to blindly set everything to auto mode.

Remove the get_frontend(), as the code does that already,
and put the frontend changes at set_frontend, where it
belongs.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>