of: property: Make 'no port node found' output a debug message

There are cases where an unavailable port is not an error, making this
error message a false-positive. Since commit d56de8c9a17d8 ("usb: typec:
tcpm: try to get role switch from tcpc fwnode") the role switch is tried
on the port dev first and tcpc fwnode afterwards. If using the latter
bindings getting from port dev fails every time. The kernel log is flooded
with the messages like:
OF: graph: no port node found in /soc@0/bus@42000000/i2c@42530000/usb-typec@50
Silence this message by making it a debug message.

Signed-off-by: Alexander Stein <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

drivers/of/property.c

···
 		of_node_put(node);
 
 		if (!port) {
-			pr_err("graph: no port node found in %pOF\n", parent);
+			pr_debug("graph: no port node found in %pOF\n", parent);
 			return NULL;
 		}
 	} else {