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