[media] saa7115: move the autodetection code out of the probe function

As we're now seeing other variants from chinese clones, like
gm1113c, we'll need to add more bits at the detection code.
So, move it into a separate function.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Tested-by: Jon Arne Jørgensen <jonarne@jonarne.no>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>