acpi: fix integer as NULL pointer warning

drivers/acpi/dispatcher/dsmethod.c:568:50: warning: Using plain integer as NULL pointer
drivers/acpi/executer/exmutex.c:329:30: warning: Using plain integer as NULL pointer
drivers/acpi/executer/exmutex.c:466:31: warning: Using plain integer as NULL pointer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/acpi/dispatcher/dsmethod.c

@@ -565,7 +565,7 @@ acpi_ds_terminate_control_method(union acpi_operand_object *method_desc,
 
 				acpi_os_release_mutex(method_desc->method.
 						      mutex->mutex.os_mutex);
-				method_desc->method.mutex->mutex.thread_id = 0;
+				method_desc->method.mutex->mutex.thread_id = NULL;
 			}
 		}
 

drivers/acpi/executer/exmutex.c

@@ -326,7 +326,7 @@ acpi_status acpi_ex_release_mutex_object(union acpi_operand_object *obj_desc)
 
 	/* Clear mutex info */
 
-	obj_desc->mutex.thread_id = 0;
+	obj_desc->mutex.thread_id = NULL;
 	return_ACPI_STATUS(status);
 }
 
@@ -463,7 +463,7 @@ void acpi_ex_release_all_mutexes(struct acpi_thread_state *thread)
 		/* Mark mutex unowned */
 
 		obj_desc->mutex.owner_thread = NULL;
-		obj_desc->mutex.thread_id = 0;
+		obj_desc->mutex.thread_id = NULL;
 
 		/* Update Thread sync_level (Last mutex is the important one) */