MIPS: ftrace: Remove pointer comparison to 0 in prepare_ftrace_return

Replace pointer comparison to 0 with NULL in prepare_ftrace_return
to improve code readability. Identified with coccinelle script
'badzero.cocci'.

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18494/


Signed-off-by: James Hogan <jhogan@kernel.org>