[AX25] ax25_ds_timer: use mod_timer instead of add_timer

This patch changes current use of: init_timer(), add_timer()
and del_timer() to setup_timer() with mod_timer(), which
should be safer anyway.

Reported-by: Jann Traschewski <jann@gmx.de>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ax25.h b/include/net/ax25.h
index 3f0236f..717e219 100644
--- a/include/net/ax25.h
+++ b/include/net/ax25.h
@@ -324,6 +324,7 @@
 extern void ax25_dama_off(ax25_cb *);
 
 /* ax25_ds_timer.c */
+extern void ax25_ds_setup_timer(ax25_dev *);
 extern void ax25_ds_set_timer(ax25_dev *);
 extern void ax25_ds_del_timer(ax25_dev *);
 extern void ax25_ds_timer(ax25_cb *);