V4L/DVB (12521): soc-camera: use .s_std() from struct v4l2_subdev_core_ops

Remove .set_std() method from struct soc_camera_ops, use .s_std() from
struct v4l2_subdev_core_ops instead.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>