[Blackfin] arch: current_l1_stack_save is a pointer, so use NULL rather than 0

Signed-off-by: NMike Frysinger <vapier.adi@gmail.com>
Signed-off-by: NBryan Wu <cooloney@kernel.org>

include/asm-blackfin/mmu_context.h

@@ -73,7 +73,7 @@ static inline void destroy_context(struct mm_struct *mm)
 	struct sram_list_struct *tmp;
 
 	if (current_l1_stack_save == mm->context.l1_stack_save)
-		current_l1_stack_save = 0;
+		current_l1_stack_save = NULL;
 	if (mm->context.l1_stack_save)
 		free_l1stack();