iwlwifi: clean up and bug fix for security

This patch cleans up code in security.

1) uses the new pointer to ieee80211_key_conf passed with the tx_control.
2) resolves bug reported by Mirco Tischler (sends ADD_STA in ASYNC mode)
3) resolves bug reported by Volker Braun regarding dynamic WEP
4) drops a WEP packet which has been garbaged by firmware. This can
happen upon rekeying.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>