xen/swiotlb: Fix compile warnings when using plain integer instead of NULL pointer.

arch/x86/xen/pci-swiotlb-xen.c:96:1: warning: Using plain integer as NULL pointer
arch/x86/xen/pci-swiotlb-xen.c:96:1: warning: Using plain integer as NULL pointer
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/pci-swiotlb-xen.c

@@ -63,6 +63,6 @@ void __init pci_xen_swiotlb_init(void)
 	}
 }
 IOMMU_INIT_FINISH(pci_xen_swiotlb_detect,
-		  0,
+		  NULL,
 		  pci_xen_swiotlb_init,
-		  0);
+		  NULL);