V4L/DVB (9318): v4l2-int-if: Add command to get slave private data.

vidioc_int_g_priv is used to get master's slave-related private data
structure. The structure can contain for example master's configuration
specific to slave.

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