net: dsa: Ensure validity of dst->ds[0]

It is perfectly possible to have non zero indexed switches being present
in a DSA switch tree, in such a case, we will be deferencing a NULL
pointer while dsa_cpu_port_ethtool_{setup,restore}. Be more defensive
and ensure that dst->ds[0] is valid before doing anything with it.

Fixes: 0c73c523 ("net: dsa: Initialize CPU port ethtool ops per tree")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa2.c

@@ -394,9 +394,11 @@ static int dsa_dst_apply(struct dsa_switch_tree *dst)
 			return err;
 	}
 
+	if (dst->ds[0]) {
 		err = dsa_cpu_port_ethtool_setup(dst->ds[0]);
 		if (err)
 			return err;
+	}
 
 	/* If we use a tagging format that doesn't have an ethertype
 	 * field, make sure that all packets from this point on get
@@ -433,6 +435,7 @@ static void dsa_dst_unapply(struct dsa_switch_tree *dst)
 		dsa_ds_unapply(dst, ds);
 	}
 
+	if (dst->ds[0])
 		dsa_cpu_port_ethtool_restore(dst->ds[0]);
 
 	pr_info("DSA: tree %d unapplied\n", dst->tree);