iwmc3200wifi: fix NULL pointer dereference in iwm_if_free

The driver private data is now based on wiphy. So we should not
touch the private data after wiphy_free() is called. The patch
fixes the potential NULL pointer dereference by making the
iwm_wdev_free() the last one on the interface removal path.

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