patches for if_iwi and wlan for WEP mode

Hi,

I've fixed iwi(4) so that Intel(R) PRO/Wireless 2915ABG work
in WEP mode, which seems to be broken since 8.0-RELEASE.

The patches against HEAD at:
http://people.freebsd.org/~iwasaki/iwi/iwi-20120305.diff

I'm not sure that changes on ieee80211 layer are right fixes,
but all of mbufs were discarded in ieee80211_start() in WEP mode.

----
void
ieee80211_start(struct ifnet *ifp)
{
[snip]
                        if (ni->ni_associd == 0 &&
                            (ni->ni_flags & IEEE80211_NODE_ASSOCID)) {
                                IEEE80211_DISCARD_MAC(vap, IEEE80211_MSG_OUTPUT,
                                    eh->ether_dhost, NULL,
                                    "sta not associated (type 0x%04x)",
                                    htons(eh->ether_type));
                                vap->iv_stats.is_tx_notassoc++;
                                ifp->if_oerrors++;
                                m_freem(m);
                                ieee80211_free_node(ni);
                                continue;
                        }
----

My patches set IEEE80211_NODE_ASSOCID bit only if ni->ni_associd
is set.  Any suggestions on this part are welcome.

I'm going to commit the changes coming weekend.

Thanks!