[AMD-IOMMU] amd_iommu.c device_nb should be static

[Jaswinder Singh Rajput <jaswinder@kernel.org>]

device_nb is used only by same file, it should be static.

Also fixed NULL pointer issue.

Fixed following sparse warnings :
  arch/x86/kernel/amd_iommu.c:1195:23: warning: symbol 'device_nb' was not declared. Should it be static?
  arch/x86/kernel/amd_iommu.c:1766:10: warning: Using plain integer as NULL pointer

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
---
 arch/x86/kernel/amd_iommu.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/amd_iommu.c b/arch/x86/kernel/amd_iommu.c
index 9372f04..6c99f50 100644
--- a/arch/x86/kernel/amd_iommu.c
+++ b/arch/x86/kernel/amd_iommu.c
@@ -1192,7 +1192,7 @@ out:
 	return 0;
 }
 
-struct notifier_block device_nb = {
+static struct notifier_block device_nb = {
 	.notifier_call = device_change_notifier,
 };
 
@@ -1763,7 +1763,7 @@ static void *alloc_coherent(struct device *dev, size_t size,
 	flag |= __GFP_ZERO;
 	virt_addr = (void *)__get_free_pages(flag, get_order(size));
 	if (!virt_addr)
-		return 0;
+		return NULL;
 
 	paddr = virt_to_phys(virt_addr);
 
-- 
1.6.0.6