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>
diff --git a/drivers/acpi/executer/exmutex.c b/drivers/acpi/executer/exmutex.c
index c873ab4..a8bf3d7 100644
--- a/drivers/acpi/executer/exmutex.c
+++ b/drivers/acpi/executer/exmutex.c
@@ -326,7 +326,7 @@
 
 	/* Clear mutex info */
 
-	obj_desc->mutex.thread_id = 0;
+	obj_desc->mutex.thread_id = NULL;
 	return_ACPI_STATUS(status);
 }
 
@@ -463,7 +463,7 @@
 		/* 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) */