[media] v4l: Allow calling v4l2_device_register_subdev_nodes() multiple times

Previously multiple calls to v4l2_device_register_subdev_nodes() ended up
corrupting memory and leaking some, too. This patch changes the behaviour so
that sub-devices the device nodes of which are already registered are
ignored.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>