Input: synaptics - simplify pass-through port handling

There was too much knowledge about internals if serio in the pass-through
handling, clean it up.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
diff --git a/drivers/input/mouse/synaptics.h b/drivers/input/mouse/synaptics.h
index b6aa7d2..613a365 100644
--- a/drivers/input/mouse/synaptics.h
+++ b/drivers/input/mouse/synaptics.h
@@ -110,6 +110,8 @@
 	unsigned char pkt_type;			/* packet type - old, new, etc */
 	unsigned char mode;			/* current mode byte */
 	int scroll;
+
+	struct serio *pt_port;			/* Pass-through serio port */
 };
 
 void synaptics_module_init(void);