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: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/dsa.h

@@ -190,7 +190,7 @@ struct dsa_port {
 	struct dsa_switch	*ds;
 	unsigned int		index;
 	const char		*name;
-	struct dsa_port		*cpu_dp;
+	const struct dsa_port	*cpu_dp;
 	struct device_node	*dn;
 	unsigned int		ageing_time;
 	u8			stp_state;

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;