staging: vt6655: device_main: Replace NULL comparison with !x

Replace NULL comparison to resolve checkpatch issue.
- x == NULL => !x
Signed-off-by: Namrata A Shettar <namrataashettar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vt6655/device_main.c

@@ -477,7 +477,7 @@ static bool device_init_rings(struct vnt_private *priv)
 					     CB_MAX_BUF_SIZE,
 					     &priv->tx_bufs_dma0,
 					     GFP_ATOMIC);
-	if (priv->tx0_bufs == NULL) {
+	if (!priv->tx0_bufs) {
 		dev_err(&priv->pcid->dev, "allocate buf dma memory failed\n");
 
 		dma_free_coherent(&priv->pcid->dev,