[AX25] ax25_timer: use mod_timer instead of add_timer

According to one of Jann's OOPS reports it looks like
BUG_ON(timer_pending(timer)) triggers during add_timer()
in ax25_start_t1timer(). 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>