[SCSI] qla2xxx: Fix race that could hang kthread_stop()

There is a small race window in qla2x00_do_dpc() between
checking for kthread_should_stop() and going to sleep after
setting TASK_INTERRUPTIBLE. If qla2x00_free_device() is called
in this window, kthread_stop will wait forever because there
will be no one to wake up the process.

Fix by making sure we only set TASK_INTERRUPTIBLE before checking
kthread_stop().

Reported-by: Bandan Das <bandan.das@stratus.com>
Acked-by: Madhuranath Iyengar <Madhu.Iyengar@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>