[media] V4L: add .g_std() core V4L2 subdevice operation

VIDIOC_G_STD can return the current TV-norm to the user in one of two ways:
if an .vidioc_g_std() ioctl operation is provided by the driver, it is
called, otherwise the value ot the .current_norm field of struct
video_device is returned. Since subdevice drivers currently have no access
to struct video_device objects, the only way to provide this information to
the user is by implementing a .g_std() method.

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

include/media/v4l2-subdev.h

@@ -158,6 +158,7 @@ struct v4l2_subdev_core_ops {
 	int (*s_ext_ctrls)(struct v4l2_subdev *sd, struct v4l2_ext_controls *ctrls);
 	int (*try_ext_ctrls)(struct v4l2_subdev *sd, struct v4l2_ext_controls *ctrls);
 	int (*querymenu)(struct v4l2_subdev *sd, struct v4l2_querymenu *qm);
+	int (*g_std)(struct v4l2_subdev *sd, v4l2_std_id *norm);
 	int (*s_std)(struct v4l2_subdev *sd, v4l2_std_id norm);
 	long (*ioctl)(struct v4l2_subdev *sd, unsigned int cmd, void *arg);
 #ifdef CONFIG_VIDEO_ADV_DEBUG