[media] DVB: Point to the generic error chapter

Just like the V4L, MC and LIRC API's, point to the generic error
chapter for ioctl's. This will allow moving generic error codes
to just one place inside all media API's.

A latter patch will remove the generic errors from each specific
ioctl.

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