net: stmmac: Rewrite two test against NULL value

This patch rewrite two test against NULL value with correct style.
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -689,7 +689,7 @@ static void stmmac_adjust_link(struct net_device *dev)
 	int new_state = 0;
 	unsigned int fc = priv->flow_ctrl, pause_time = priv->pause;
 
-	if (phydev == NULL)
+	if (!phydev)
 		return;
 
 	spin_lock_irqsave(&priv->lock, flags);
@@ -1131,7 +1131,7 @@ static void dma_free_tx_skbufs(struct stmmac_priv *priv)
 						 DMA_TO_DEVICE);
 		}
 
-		if (priv->tx_skbuff[i] != NULL) {
+		if (priv->tx_skbuff[i]) {
 			dev_kfree_skb_any(priv->tx_skbuff[i]);
 			priv->tx_skbuff[i] = NULL;
 			priv->tx_skbuff_dma[i].buf = 0;