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/usb/misc/sisusbvga/sisusb_con.c

@@ -717,24 +717,22 @@ sisusbcon_blank(struct vc_data *c, int blank, int mode_switch)
 }
 
 /* interface routine */
-static int
+static void
 sisusbcon_scrolldelta(struct vc_data *c, int lines)
 {
 	struct sisusb_usb_data *sisusb;
 	int margin = c->vc_size_row * 4;
 	int ul, we, p, st;
 
-	/* The return value does not seem to be used */
-
 	sisusb = sisusb_get_sisusb_lock_and_check(c->vc_num);
 	if (!sisusb)
-		return 0;
+		return;
 
 	/* sisusb->lock is down */
 
 	if (sisusb_is_inactive(c, sisusb)) {
 		mutex_unlock(&sisusb->lock);
-		return 0;
+		return;
 	}
 
 	if (!lines)		/* Turn scrollback off */
@@ -774,8 +772,6 @@ sisusbcon_scrolldelta(struct vc_data *c, int lines)
 	sisusbcon_set_start_address(sisusb, c);
 
 	mutex_unlock(&sisusb->lock);
-
-	return 1;
 }
 
 /* Interface routine */
@@ -1433,7 +1429,6 @@ static const struct consw sisusb_dummy_con = {
 	.con_font_default =	SISUSBCONDUMMY,
 	.con_font_copy =	SISUSBCONDUMMY,
 	.con_set_palette =	SISUSBCONDUMMY,
-	.con_scrolldelta =	SISUSBCONDUMMY,
 };
 
 int