iwlagn: fix HW crypto for TX-only keys

Group keys in IBSS or AP mode are not programmed
into the device since we give the key to it with
every TX packet. However, we do need mac80211 to
create the MMIC & PN in all cases. Move the code
around to set the key flags all the time. We set
them even when the key is removed again but that
is obviously harmless.

Cc: stable@vger.kernel.org
Reported-by: Reinette Chatre <reinette.chatre@intel.com>
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>