[media] tuner-core: Reorganize the functions internally

This is a big patch with no functional changes. It just
rearranges everything inside the driver, and prepares to
break TV and Radio into two separate fops groups.

Currently, it has an heuristics logic to determine if the
call came from radio or video. However, the caller driver
knows for sure, so tuner-core shouldn't try to guess it.

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