iwlwifi: unregister to upper stack before releasing resources

This patch fixes an early release of driver's resources before upper stack
was notified that low-level driver shuts down.

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>