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>

drivers/tty/vt/vt.c

@@ -2468,7 +2468,7 @@ static void console_callback(struct work_struct *ignored)
 	if (scrollback_delta) {
 		struct vc_data *vc = vc_cons[fg_console].d;
 		clear_selection();
-		if (vc->vc_mode == KD_TEXT)
+		if (vc->vc_mode == KD_TEXT && vc->vc_sw->con_scrolldelta)
 			vc->vc_sw->con_scrolldelta(vc, scrollback_delta);
 		scrollback_delta = 0;
 	}