drivers/net/netxen: fix sparse warnings: use NULL pointer instead of plain integer

Fix this sparse warnings:

  drivers/net/netxen/netxen_nic_hw.c:1462:18: warning: Using plain integer as NULL pointer
  drivers/net/netxen/netxen_nic_hw.c:1536:18: warning: Using plain integer as NULL pointer
Signed-off-by: NHannes Eder <hannes@hanneseder.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/netxen/netxen_nic_hw.c

@@ -1459,7 +1459,7 @@ static int netxen_nic_pci_mem_read_direct(struct netxen_adapter *adapter,
 			mem_ptr = ioremap(mem_base + mem_page, PAGE_SIZE * 2);
 		else
 			mem_ptr = ioremap(mem_base + mem_page, PAGE_SIZE);
-		if (mem_ptr == 0UL) {
+		if (mem_ptr == NULL) {
 			*(uint8_t  *)data = 0;
 			return -1;
 		}
@@ -1533,7 +1533,7 @@ netxen_nic_pci_mem_write_direct(struct netxen_adapter *adapter, u64 off,
 			mem_ptr = ioremap(mem_base + mem_page, PAGE_SIZE*2);
 		else
 			mem_ptr = ioremap(mem_base + mem_page, PAGE_SIZE);
-		if (mem_ptr == 0UL)
+		if (mem_ptr == NULL)
 			return -1;
 		addr = mem_ptr;
 		addr += start & (PAGE_SIZE - 1);