Input: synaptics - fix compile warning

Move synaptics_invert_y() inside CONFIG_MOUSE_PS2_SYNAPTICS to get rid of
a compile warning when we don't select synaptics support.

drivers/input/mouse/synaptics.c:53:12: warning: ‘synaptics_invert_y’ defined but not used [-Wunused-function]
Signed-off-by: JJ Ding <dgdunix@gmail.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/mouse/synaptics.c

@@ -45,16 +45,6 @@
 #define YMIN_NOMINAL 1408
 #define YMAX_NOMINAL 4448
 
-/*
- * Synaptics touchpads report the y coordinate from bottom to top, which is
- * opposite from what userspace expects.
- * This function is used to invert y before reporting.
- */
-static int synaptics_invert_y(int y)
-{
-	return YMAX_NOMINAL + YMIN_NOMINAL - y;
-}
-
 
 /*****************************************************************************
  *	Stuff we need even when we do not want native Synaptics support
@@ -111,6 +101,16 @@ void synaptics_reset(struct psmouse *psmouse)
  *	Synaptics communications functions
  ****************************************************************************/
 
+/*
+ * Synaptics touchpads report the y coordinate from bottom to top, which is
+ * opposite from what userspace expects.
+ * This function is used to invert y before reporting.
+ */
+static int synaptics_invert_y(int y)
+{
+	return YMAX_NOMINAL + YMIN_NOMINAL - y;
+}
+
 /*
  * Send a command to the synpatics touchpad by special commands
  */