atm: ambassador: use NULL instead of 0 for pointer

sparse says:

drivers/atm/ambassador.c:1928:24: warning:
 Using plain integer as NULL pointer
Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Acked-by: Chas Williams <chas@cmf.nrl.navy.mil>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/atm/ambassador.c

@@ -1925,7 +1925,7 @@ static int ucode_init(loader_block *lb, amb_dev *dev)
   const struct firmware *fw;
   unsigned long start_address;
   const struct ihex_binrec *rec;
-  const char *errmsg = 0;
+  const char *errmsg = NULL;
   int res;
 
   res = request_ihex_firmware(&fw, "atmsar11.fw", &dev->pci_dev->dev);