[media] saa7115: add config flag to change the IDQ polarity

Needed by the go7007 driver: it assumes a different polarity of the IDQ
signal, so we need to be able to tell the saa7115 about this.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>