staging: vt6655: Remove NULL pointer sparse warning

We were using 0 instead of NULL to initialize a pointer, which caused
a sparse warning.

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vt6655/device_main.c

@@ -1626,7 +1626,7 @@ static void device_free_tx_buf(PSDevice pDevice, PSTxDesc pDesc)
 		dev_kfree_skb_irq(skb);
 
 	pTDInfo->skb_dma = 0;
-	pTDInfo->skb = 0;
+	pTDInfo->skb = NULL;
 	pTDInfo->byFlags = 0;
 }