net: dsa: Fix inverted test for multiple CPU interface

Remove the wrong !, otherwise we get false positives about having
multiple CPU interfaces.

Fixes: b22de49086 ("net: dsa: store CPU switch structure in the tree")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa.c

@@ -225,7 +225,7 @@ static int dsa_switch_setup_one(struct dsa_switch *ds, struct device *parent)
 			continue;
 
 		if (!strcmp(name, "cpu")) {
-			if (!dst->cpu_switch) {
+			if (dst->cpu_switch) {
 				netdev_err(dst->master_netdev,
 					   "multiple cpu ports?!\n");
 				return -EINVAL;