x86/iommu: Use NULL instead of 0

Fixes the following sparse warning:

arch/x86/kernel/pci-iommu_table.c:63:37: warning: Using plain integer as NULL pointer
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <hpa@zytor.com>
Cc: <gregkh@linuxfoundation.org>
Link: https://lkml.kernel.org/r/1532162004-24670-1-git-send-email-zhongjiang@huawei.com

arch/x86/kernel/pci-iommu_table.c

@@ -60,7 +60,7 @@ void __init check_iommu_entries(struct iommu_table_entry *start,
 			printk(KERN_ERR "CYCLIC DEPENDENCY FOUND! %pS depends on %pS and vice-versa. BREAKING IT.\n",
 			       p->detect, q->detect);
 			/* Heavy handed way..*/
-			x->depend = 0;
+			x->depend = NULL;
 		}
 	}