[SCSI] bnx2fc: fix panic in bnx2fc_post_io_req

System panics while accessing stale pointer - timer_work_queue - in the IO path
before bnx2fc_stop is called. Fix is to destroy the workqueue after the destroy
operation is complete.

Signed-off-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>