iwlwifi: mvm: fix TKIP key updating

When a TKIP key is updated with a station pointer that is NULL it is
a GTK, so it should use the AP's station ID. Fix the code to do that.

Signed-off-by: Beni Lev <beni.lev@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>