V4L/DVB (9213): si470x: module_param access rights

This patch mainly adds correct module_param access rights. Also there
are a lot of small coding style enhancements and some corrections of the
variable references in module_param.

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