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

[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 <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>