net: dsa: constify cpu_dp member of dsa_port

A DSA port has a dedicated CPU port assigned to it, stored in the cpu_dp
member. It is not meant to be modified by a port, thus make it const.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/dsa/slave.c b/net/dsa/slave.c
index 814ced7..cc7fe47 100644
--- a/net/dsa/slave.c
+++ b/net/dsa/slave.c
@@ -1147,7 +1147,7 @@ static void dsa_slave_notify(struct net_device *dev, unsigned long val)
 
 int dsa_slave_create(struct dsa_port *port)
 {
-	struct dsa_port *cpu_dp = port->cpu_dp;
+	const struct dsa_port *cpu_dp = port->cpu_dp;
 	struct net_device *master = cpu_dp->master;
 	struct dsa_switch *ds = port->ds;
 	const char *name = port->name;