net: dsa: print error on invalid port index

Looking for an -EINVAL all over the dsa code could take hours for
inexperienced DSA users.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210106090915.21439-1-zajec5@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/dsa2.c

@@ -783,6 +783,8 @@ static int dsa_switch_parse_ports_of(struct dsa_switch *ds,
 			goto out_put_node;
 
 		if (reg >= ds->num_ports) {
+			dev_err(ds->dev, "port %pOF index %u exceeds num_ports (%zu)\n",
+				port, reg, ds->num_ports);
 			err = -EINVAL;
 			goto out_put_node;
 		}