media: v4l2-fwnode: Return -EPROBE_DEFER on parsing NULL endpoints

media: v4l2-fwnode: Return -EPROBE_DEFER on parsing NULL endpoints

In general drivers get their firmware graph endpoints from system
firmware, but on some systems this information is conveyed to drivers via
software nodes. The software nodes may be instantiated only after the
drivers are first probed, requiring drivers to explicitly issue
-EPROBE_DEFER when endpoints aren't found.

Instead of doing this in all (or at least most) drivers, make v4l2-fwnode
endpoint parsing functions v4l2_fwnode_endpoint_parse() and
v4l2_fwnode_endpoint_alloc_parse() return -EPROBE_DEFER when an endpoint
is NULL.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>