[media] si470x: Clean up, introduce the control framework

This cleans up the code and si470x now uses the proper v4l2 frameworks
and passes most of the v4l2-compliance tests.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>