V4L/DVB (9214): si470x: improvement of module device support

This patch improves support for multiple radio devices.

In previous versions all region relevant settings were derived from one
module parameter. As in future versions, the region and other
configuration should be configurable per device from the user space,
this patch already retrieves all relevant information from the actual
device specific settings.

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