xirc2ps_cs: Fix use after free bug in xirc2ps_detach

In xirc2ps_probe, the local->tx_timeout_task was bounded
with xirc2ps_tx_timeout_task. When timeout occurs,
it will call xirc_tx_timeout->schedule_work to start the
work.

When we call xirc2ps_detach to remove the driver, there
may be a sequence as follows:

Stop responding to timeout tasks and complete scheduled
tasks before cleanup in xirc2ps_detach, which will fix
the problem.

CPU0                  CPU1

                    |xirc2ps_tx_timeout_task
xirc2ps_detach      |
  free_netdev       |
    kfree(dev);     |
                    |
                    | do_reset
                    |   //use dev

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Zheng Wang <zyytlz.wz@163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
