iwmc3200wifi: fix potential kernel oops on module removal

The iwm_if_free() is called before destroy_workqueue for isr_wq on
device remove method. But if there is still some pending work in
the isr_wq, the required data structures are already freed at this
point. This leeds a kernel oops. The patch fixes this problem by
moving iwm_if_free after destroy_workqueue.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Samuel Ortiz <samuel.ortiz@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>