media: vidtv: fix frequency tuning logic

Right now, there are some issues at the tuning logic:

1) the config struct is not copied at the tuner driver.
   so, it won't use any frequency table at all;
2) the code that checks for frequency shifts is called
   at set_params. So, lock_status will never be zeroed;
3) the signal strength will also report a strong
   signal, even if not tuned;
4) the logic is not excluding non-set frequencies.

Fix those issues.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>