[media] saa6588: rename rds.h to saa6588.h

The naming of the media/rds.h header suggested that it was a generic
RDS header, when in fact it is just a saa6588 module API that is
internal to the kernel.

Rename the header and the struct and defines in it to make this clear.

Also removed the header include in radio-si470x.h (not used anymore)
and from ioctl-number.txt (it's internal to the kernel and never called
from userspace).

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>