[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>