iwlagn: properly wait for PAN disable

Previously I hacked this with an msleep(300)
which was fine since we never had longer PAN
time slots, but now that we will have them I
need to fix that. Use the new notification
wait support to properly wait for the WIPAN
deactivation complete signal from the ucode.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>