ACPI: Fix sparse warnings

Use NULL for pointers

drivers/acpi/osl.c:208:10: warning: Using plain integer as NULL pointer
drivers/acpi/tables/tbxface.c:411:49: warning: Using plain integer as NULL pointer
drivers/acpi/processor_core.c:1008:10: warning: Using plain integer as NULL pointer

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
index 0f6f3bc..63060a1 100644
--- a/drivers/acpi/osl.c
+++ b/drivers/acpi/osl.c
@@ -205,7 +205,7 @@
 {
 	if (phys > ULONG_MAX) {
 		printk(KERN_ERR PREFIX "Cannot map memory that high\n");
-		return 0;
+		return NULL;
 	}
 	if (acpi_gbl_permanent_mmap)
 		/*