[media] tm6000: rework init code

Rework device init part. Move common code part to a function.
Usefull for register multiple devices like video, radio, vbi etc.

Signed-off-by: Beholder Intl. Ltd. Dmitry Belimov <d.belimov@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>