tty: vgacon+sisusb, move scrolldelta to a common helper

The code is mirrorred in scrolldelta implementations of both vgacon
and sisusb. Let's move the code to a separate helper where we will
perform a common cleanup and further changes.

While we are moving the code, make it linear and save one indentation
level. This is done by returning from the "!lines" then-branch
immediatelly. This allows flushing the else-branch 1 level to the
left, obviously.

Few more new lines and comments were added too.

And do not forget to export the helper function given sisusb can be
built as module.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: <linux-fbdev@vger.kernel.org>
Cc: <linux-usb@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/include/linux/vt_kern.h b/include/linux/vt_kern.h
index 6abd24f..833fdd4 100644
--- a/include/linux/vt_kern.h
+++ b/include/linux/vt_kern.h
@@ -191,5 +191,7 @@ extern void vt_set_led_state(int console, int leds);
 extern void vt_kbd_con_start(int console);
 extern void vt_kbd_con_stop(int console);
 
+void vc_scrolldelta_helper(struct vc_data *c, int lines,
+		unsigned int rolled_over, void *_base, unsigned int size);
 
 #endif /* _VT_KERN_H */