[MIPS] 0 -> NULL

When comparing a pointer, it's clearer to compare it to NULL than to 0.

Here is an excerpt of the semantic patch:

@@
expression *E;
@@

  E ==
- 0
+ NULL

@@
expression *E;
@@

  E !=
- 0
+ NULL

Signed-off-by: Yoann Padioleau <padator@wanadoo.fr>
Cc: ralf@linux-mips.org
Cc: linux-mips@linux-mips.org
Cc: akpm@linux-foundation.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/pci/ops-emma2rh.c

@@ -45,7 +45,7 @@ static int check_args(struct pci_bus *bus, u32 devfn, u32 * bus_num)
 	/* check if the bus is top-level */
 	if (bus->parent != NULL) {
 		*bus_num = bus->number;
-		db_assert(bus_num != 0);
+		db_assert(bus_num != NULL);
 	} else
 		*bus_num = 0;