ieee802154: socket: fix checkpatch issue

This patch solves the following checkpatch issue:

CHECK: Comparison to NULL could be written "skb"
+		if (skb != NULL) {

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
diff --git a/net/ieee802154/af_ieee802154.c b/net/ieee802154/af_ieee802154.c
index 64beb0b..2878d8c 100644
--- a/net/ieee802154/af_ieee802154.c
+++ b/net/ieee802154/af_ieee802154.c
@@ -560,7 +560,7 @@
 		amount = 0;
 		spin_lock_bh(&sk->sk_receive_queue.lock);
 		skb = skb_peek(&sk->sk_receive_queue);
-		if (skb != NULL) {
+		if (skb) {
 			/* We will only return the amount
 			 * of this packet since that is all
 			 * that will be read.