media: subdev: Require code change to enable [GS]_ROUTING

The Streams API is an experimental feature. To use the Streams API, the
user needs to change a variable in v4l2-subdev.c and recompile the kernel.

This commit should be reverted when the Streams API is deemed ready for
production use.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>