microblaze: Remove sparse error in traps.c

CHECK   arch/microblaze/kernel/traps.c
arch/microblaze/kernel/traps.c:37:47: warning: Using plain integer as NULL pointer
CC      arch/microblaze/kernel/traps.o

Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/kernel/traps.c

@@ -34,7 +34,7 @@ static int kstack_depth_to_print = 24;
 
 static int __init kstack_setup(char *s)
 {
-	kstack_depth_to_print = strict_strtoul(s, 0, 0);
+	kstack_depth_to_print = strict_strtoul(s, 0, NULL);
 
 	return 1;
 }