mlxsw: spectrum: use netif_is_macsec() instead of open code

Open code which is dev->priv_flags & IFF_MACSEC has already defined as
netif_is_macsec(). So use netif_is_macsec() instead of open code.
This patch doesn't change logic.
Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -5237,7 +5237,7 @@ static inline void netif_keep_dst(struct net_device *dev)
 static inline bool netif_reduces_vlan_mtu(struct net_device *dev)
 {
 	/* TODO: reserve and use an additional IFF bit, if we get more users */
-	return dev->priv_flags & IFF_MACSEC;
+	return netif_is_macsec(dev);
 }
 
 extern struct pernet_operations __net_initdata loopback_net_ops;