servers/app/drawing: BitmapHWInterface: Return -1.

-1 is the same as B_ERROR, but all of the other implementations of
this function "return -1", so do that to be consistent.

src/servers/app/drawing/BitmapHWInterface.cpp

@@ -144,7 +144,7 @@ BitmapHWInterface::ProposeMode(display_mode* candidate, const display_mode* low,
 sem_id
 BitmapHWInterface::RetraceSemaphore()
 {
-	return B_ERROR;
+	return -1;
 }