[media] v4l: Correct conflicting V4L2 subdev selection API documentation

The API reference documents that the KEEP_CONFIG flag tells the
configuration should not be propagated by the driver whereas the interface
documentation (dev-subdev.xml) prohibited any changes to the rest of the
pipeline. Resolve the conflict by changing the API reference to disallow
changes.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>