[media] dib8000: improves the auto search mode check logic

The logic that detects if auto search mode should be used is too
complex.

Also, it doesn't cover all cases, as the dib8000_tune logic
requires either auto mode or a fully specified manual mode.
So, move it to a separate function and add some extra debug
data to help identifying when it falled back to auto mode,
because the manual settings are invalid.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Patrick Boettcher <pboettcher@kernellabs.com>