tty: vt, consw->con_scrolldelta cleanup

* allow NULL consw->con_scrolldelta (some consoles define an empty
  hook)
* => remove empty hooks now
* return value of consw->con_scrolldelta is never checked => make the
  function void
* document consw->con_scrolldelta a bit

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Cc: linux-usb@vger.kernel.org
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: linux-parisc@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/include/linux/console.h b/include/linux/console.h
index 98c8615..d175de8 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -28,6 +28,12 @@
 #define VT100ID "\033[?1;2c"
 #define VT102ID "\033[?6c"
 
+/**
+ * struct consw - callbacks for consoles
+ *
+ * @con_scrolldelta: the contents of the console should be scrolled by @lines.
+ *		     Invoked by user. (optional)
+ */
 struct consw {
 	struct module *owner;
 	const char *(*con_startup)(void);
@@ -48,7 +54,7 @@
 	int     (*con_resize)(struct vc_data *, unsigned int, unsigned int,
 			       unsigned int);
 	int	(*con_set_palette)(struct vc_data *, const unsigned char *);
-	int	(*con_scrolldelta)(struct vc_data *, int);
+	void	(*con_scrolldelta)(struct vc_data *, int lines);
 	int	(*con_set_origin)(struct vc_data *);
 	void	(*con_save_screen)(struct vc_data *);
 	u8	(*con_build_attr)(struct vc_data *, u8, u8, u8, u8, u8, u8);