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>