net: dsa: make cpu_dp non const

A port may trigger operations on its dedicated CPU port, so using
cpu_dp as const will raise warnings. Make cpu_dp non const.
Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dsa.h

@@ -181,7 +181,7 @@ struct dsa_port {
 	struct dsa_switch	*ds;
 	unsigned int		index;
 	const char		*name;
-	const struct dsa_port	*cpu_dp;
+	struct dsa_port		*cpu_dp;
 	const char		*mac;
 	struct device_node	*dn;
 	unsigned int		ageing_time;