mac80211: Fixing slow IBSS rejoin

This patch fixes the issue of slow reconnection to an IBSS cell after
disconnection from it. Now the interface's bssid is reset upon ifdown.

ieee80211_sta_find_ibss:
if (found && memcmp(ifsta->bssid, bssid, ETH_ALEN) != 0 &&
	    (bss = ieee80211_rx_bss_get(dev, bssid,
					local->hw.conf.channel->center_freq,
					ifsta->ssid, ifsta->ssid_len)))

Note:
In general disconnection is still not handled properly in mac80211

Signed-off-by: Assaf Krauss <assaf.krauss@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>