atm: firestream: Use del_timer_sync() in teardown path

The device is about to vanish. So we need to make sure that the timer
is completely stopped and the callback is not running on another CPU.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Chas Williams <chas@cmf.nrl.navy.mil>
Cc: atm <linux-atm-general@lists.sourceforge.net>
Cc: netdev <netdev@vger.kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>