V4L/DVB (7947): videodev: add vidioc_g_std callback.

The default videodev behavior for VIDIOC_G_STD is not correct for all devices.
Add a new callback that drivers can use instead.

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