[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>