V4L/DVB (11370): v4l2-subdev: move s_std from tuner to core.

s_std didn't belong in the tuner ops. Stricly speaking it should be part of
the video ops, but it is used by audio and tuner devices as well, so it is
more efficient to make it part of the core ops.

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