V4L/DVB (9216): si470x: tuner->type handling

The V4L2 specification says, when to check and when to return
tuner->type as constant value.

This patch corrects exactly this behavior, so that it is now conform to
the V4L2 specification.

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