V4L/DVB (7996): si470x: afc indication

This patch brings the following changes:
- afc indication:
  device has no indication whether freq is too low or too high
  therefore afc always return 1, when freq is wrong

Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>