net: dsa: make dsa_slave_dev_check use const

The switchdev handle helpers make use of a device checking helper
requiring a const net_device. Make dsa_slave_dev_check compliant
to this.
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>

net/dsa/slave.c

@@ -22,7 +22,7 @@
 
 #include "dsa_priv.h"
 
-static bool dsa_slave_dev_check(struct net_device *dev);
+static bool dsa_slave_dev_check(const struct net_device *dev);
 
 /* slave mii_bus handling ***************************************************/
 static int dsa_slave_phy_read(struct mii_bus *bus, int addr, int reg)
@@ -1408,7 +1408,7 @@ void dsa_slave_destroy(struct net_device *slave_dev)
 	free_netdev(slave_dev);
 }
 
-static bool dsa_slave_dev_check(struct net_device *dev)
+static bool dsa_slave_dev_check(const struct net_device *dev)
 {
 	return dev->netdev_ops == &dsa_slave_netdev_ops;
 }