dma-debug: Fix bug causing build warning

Stephen Rothwell reported the following build warning:

 lib/dma-debug.c: In function 'dma_debug_device_change':
 lib/dma-debug.c:680: warning: 'return' with no value, in function returning non-void

Introduced by commit f797d988
("dma-debug: Do not add notifier when dma debugging is disabled").

Return 0 [notify-done] when disabled. (this is standard bus notifier behavior.)
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@kernel.org>
LKML-Reference: <20091231125624.GA14666@liondog.tnic>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

lib/dma-debug.c

@@ -670,14 +670,13 @@ static int device_dma_allocations(struct device *dev)
 	return count;
 }
 
-static int dma_debug_device_change(struct notifier_block *nb,
-				    unsigned long action, void *data)
+static int dma_debug_device_change(struct notifier_block *nb, unsigned long action, void *data)
 {
 	struct device *dev = data;
 	int count;
 
 	if (global_disable)
-		return;
+		return 0;
 
 	switch (action) {
 	case BUS_NOTIFY_UNBOUND_DRIVER: