iwlwifi: move firmware completion wait

This doesn't belong into the op_mode, it has
to be in the drv stop flow instead.

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>