skge: Adjust a null pointer check in skge_down()

The script "checkpatch.pl" pointed information out like the following.

Comparison to NULL could be written "!skge->mem".

Thus fix the affected source code place.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/skge.c

@@ -2657,7 +2657,7 @@ static int skge_down(struct net_device *dev)
 	struct skge_hw *hw = skge->hw;
 	int port = skge->port;
 
-	if (skge->mem == NULL)
+	if (!skge->mem)
 		return 0;
 
 	netif_info(skge, ifdown, skge->netdev, "disabling interface\n");