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/processor_core.c b/drivers/acpi/processor_core.c
index 61d976e..4b3d15f 100644
--- a/drivers/acpi/processor_core.c
+++ b/drivers/acpi/processor_core.c
@@ -1005,7 +1005,7 @@
 #ifdef CONFIG_SMP
 	if (ACPI_FAILURE(acpi_get_table(ACPI_SIG_MADT, 0,
 				(struct acpi_table_header **)&madt)))
-		madt = 0;
+		madt = NULL;
 #endif
 
 	acpi_processor_dir = proc_mkdir(ACPI_PROCESSOR_CLASS, acpi_root_dir);